NOAA Technical Report NMFS 105 December 1991

Biology, Oceanography, and Fisheries

of the North Pacific Transition Zone

and Subarctic Frontal Zone

Jerry A. Wetherall (editor)

u.s. Department of Commerce

<:.-

:==:::::::::=======-==-=-=....:.=======================-----=====:.....=::=::::.=........:::......:=:=:::::::::===::::::::=...-
NOAA Technical Report NMFS

The major responsibilities ofthe National Marine Fisheries Service (NMFS) are to monitor and assess the abundance and geographic distribution of fishery resources,

to understand and predict fluctuations in the quantity

and distribution of these resources, and to establish levels

for their optimum use. NMFS is also charged with the

development and implementation of policies for managing national fishing grounds, with the development and

enforcement of domestic fisheries regulations, with the

surveillance offoreign fishing off U.S. coastal waters, and

with the development and enforcement of international

fishery agreements and policies. NMFS also assists the

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and "Circular." The series contains the following types of

reports: scientific investigations that document long-term

continuing programs ofNMFS; intensive scientific reports

on studies of restricted scope; papers on applied fishery

problems; technical reports of general interest intended

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certain broad areas ofresearch; and technical papers originating in economic studies and in management investigations. Since this is a formal series, all submitted papers,

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receive peer review and all papers, once accepted, receive professional editing before publication.

Copies of NOAA Technical Reports NMFS are available free in limited numbers to government agencies,

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Recently Published NOAA Technical Reports NMFS

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edited by Harold L. PrattJr., Samuel H. Gruber, and Toru

Taniuchi. July 1990, 518 p.

91. Marine flora and fauna ofthe northeastern United StatesEchinodermata: Crinoidea, by Charles G. Messing and

John H. Dearborn. August 1990, 30 p.

92. Genetics in aquaculture: proceedings of the sixteenth

U.S.-Japan meeting on aquaculture; Charleston, South

Carolina, 20-21 October, 1987, edited by Ralph S. SVIjcek.

November 1990, 81 p.

93. Distribution and abundance ofjuvenile salmonids off Oregon and Washington, 1981-1985, by William G. Pearcy

and Joseph P. Fisher. November 1990, 83 p.

94. An economics guide to allocation of f'lSh stocks between

commercial and recreational fisheries, by Steven F.

Edwards. November 1990, 29 p.

95. Larval f'1Sh recruitment and research in the Americas: proceedings of the thirteenth annual larval f'lSh conference;

Merida, Mexico, 21-26 May 1989, edited by Robert D.

Hoyt. January 1991,147 p.

96. Marine flora and fauna of the Eastern United States--

Copepoda, Cyclopoida: Archinotodelphyidae,

Notodeiphyidae, and Ascidicolidae, by Patricia L. Dudley

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97. Catalog of osteological collections of aquatic mammals

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98. Marine mammal strandings in the United States: proceedings of the second marine mammal stranding workshop; Miami, Florida, 3-5 December, 1987, edited byJohn

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99. Marine flora and fauna of the Northeastern United States:

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100. Marine flora and fauna of the Eastern United States:

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101. Larvae of nearshore f'lShes in oceanic waters near Oahu,

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102. Marine ranching: proceedings of the seventeenth U.S.-

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1988, edited by Ralph S. SVIjcek. May 1991,180 p.

103. Benthic macrofauna of the New York Bight, 1979-89, by

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December 1991, 57 p.
NOAA Technical Report NMFS 105

Biology, Oceanography, and Fisheries

of the North Pacific Transition Zone

and Subarctic Frontal Zone

Papersfrom the North Pacific

Transition Zone Workshop

Honolulu, Hawaii

9-11 May 1988

Jerry A. Wetherall (editor)

Sponsored by

Honolulu Laboratory

Southwest Fisheries Science Center

National Marine Fisheries Service

National Oceanic and Atmospheric Administration

2570 Dole Street

Honolulu, HI 96822

and

NOAA/University of HawaiiJoint Institute

for Marine and Atmospheric Research

1000 Pope Road

University of Hawaii

Honolulu, HI 96822

December 1991

U.S. DEPARTMENT OF COMMERCE

Robert Mosbacher, Secretary

National Oceanic and Atmospheric Administration

John A. Knauss, Under Secretary for Oceans and Atmosphere

National Marine Fisheries Service

William W. Fox Jr., Assistant Administrator for Fisheries
The National Marine Fisheries Service ( MFS) does not approve, recommend or endorse any proprietary product or proprietary material mentioned in this publication. No reference

shall be made to NMFS, or to this publication furnished by

NMFS, in any advertising or sales promotion which would indicate or imply that NMFS approves, recommends or endorses

any proprietary product or proprietary material mentioned

herein, or which has '.s its purpose an intent to cause directly

or indirectly the advertised product to be used or purchased

because of this NMFS publication.

11
Preface

G.!. RODEN

w.G. PEARCY

E.H. SINCLAIR

R.M.lAURS

RJ. LYNN

S.IGNELL

S.IGNELL

Contents

Subarctic-subtropical transitIOn zone of the North Pacific: large-scale

aspects and mesoscale structure

Biology of the transition region

Review of the biology and distribution ofthe neon flying squid (Omrnastrephes

bartrami) in the North Pacific Ocean

North Pacific albacore ecology and oceanography

Zonal variability in salmonid driftnet catch rates in the central North

Pacific Ocean

The fisheries for neon flying squid (Ommastrephes Bartrami )

in the central North Pacific Ocean

III

IV

1

39

57

69

89

97
Preface

In the past few years, large-scale, high-seas driftnet fishing has sparked intense debate and political conflict in

many oceanic regions. In the Pacific Ocean the driftnet

controversy first emerged in the North Pacific transition

zone and subarctic frontal zone, where driftnet vessels

from Japan, the Republic of Korea, and Taiwan pursue

their target species of neon flying squid. Other North

Pacific driftnet fleets fromJapan and Taiwan target stocks

of tunas and billfishes. Both types of driftnet fishing incidentally kill valued non-target species of marine life, including fish, mammals, birds, and turtles.

In response to public concerns about driftnet fishing,

government scientists began early on to assemble available information and consider what new data were required to assess impacts on North Pacific marine resources

and the broader pelagic ecosystem. Accordingly, a workshop was convened at the NMFS Honolulu Laboratory

in May 1988 to review current information on the biology, oceanography, and fisheries of the North Pacific

IV

transition zone and subarctic frontal zone. The workshop

participants, from the United States and Canada, also

developed a strategic plan to guide NMFS in developing

a program of driftnet fishery research and impact assessment.

This volume contains a selection of scientific review

papers presented at the 1988 Honolulu workshop. The

papers represent part of the small kernel of information

available then, prior to the expansion of cooperative international scientific programs. Subsequent driftnet fishery monitoring and research by the United States, Canada,

Japan, Korea, and Taiwan have added much new data.

Nevertheless, this collection of papers provides a historical perspective and contains useful information not readily

available elsewhere.

Jerry A Wetherall

Honolulu, August 1991
Subarctic-SubtropicalTransition Zone of the North Pacific:

Large-Scale Aspects and Mesoscale Structure

GUNNAR I. RODEN

School ojOceanography, WB-10

University oj Washington

Seattle, WA 98195

ABSTRACT

The subarctic-subtropical transition zone of the North Pacific is investigated on the basis

of high resolution hydrographic data. The transition zone is an oceanwide feature associated with the oceanic and atmospheric gyre scale circulations. It is bounded in the north

and south by frontal zones, in which one or several meandering fronts and frontal eddies

occur. The positions of the transition and frontal zones do not vary by more than

150-200 km from their climatological mean positions. Individual fronts are highly variable

in time and space. The northern boundary of the transition zone is marked by the disappearance of the subarctic halocline and the overlying shallow temperature minimum, and

by large changes in the hydrostatic stability structure. A seasonal stability gap develops there

in winter. The interannual variability of the subarctic front depends upon the flow dynamics at the particular geographical location. Over the Emperor Seamounts, the front appears

to be topographically trapped, varying by not more than 50 km from its mean position at

43° N during 1981-1986; elsewhere, the displacements from the mean are about 150 km.

The southern boundary of the transition zone is best described by the disappearance of the

subtropical halocline and by large changes in the structure of the thermocline. A pervasive

mesoscale eddy field is always present within the transition zone. The amplitudes of these

eddies are three times larger west than east of the Emperor Seamounts.

Introduction _

The subarctic-subtropical transition zone is one of the

main oceanic features of the North Pacific. It lies between the subarctic and subtropical gyres and extends

from Japan to North America, a distance of nearly

9000 km (Fig. 1). Its average meridional extent in midocean is about 1000 km. The northern and southern

boundaries of the transition zone have the character of

frontal zones, in which there are rapid changes in thermohaline structure, hydrostatic stability structur.e, and

biological species composition. The climatological

mean positions of the transition zone boundaries are at

latitudes 32°N and 42°N. In the central North Pacific,

the typical subarctic domain with its cool and low salinity surface water disappears within less than 150 km of

42°N (Dodimead et al. 1963; Uda 1963; Roden 1970).

As one proceeds southward across the transition zone,

temperatures and salinities gradually increase. Then,

within 150 km of 32°N, there is a rapid change to the

subtropical domain with its warm and salty top layer

(Sverdrup et al. 1942; Seckel 1968; Roden 1970, 1980).

The frontal zones are of biological interest also, because they delineate species distributions. Thus, the

copepods Euphausia pacifica and longipes occur only

north of the subarctic boundary, Euphausia brevis is

found only south of the subtropical boundary, while

Thysanoessa gregana is encountered almost exclusively in

the transition zone between the subarctic and subtropical domains (Brinton 1962). Fishermen, too, are attracted to frontal zones because of improved fishing

there.

The transition zone is the result of the large scale

ocean circulation and air-sea energy exchange and

some progress has been made in understanding the

dynamics and structure of this zone and the associated

frontal zones (Roden 1975, 1977a, 1981; Roden and

Paskausky 1978; Camerlengo 1982; Cushman-Roisin

1981; Niiler and Reynolds 1984; Rhines 1986; Roden

and Robinson 1988). The picture that emerges is one

of subtle balances among mixing, advection, diffusion,

and atmospherically induced surface heat, salt, and

momentum fluxes which lead to thermohaline structures that occur in geographically preferred regions.
2 NOAA Technical Report #105

50°

N

1200

1200

1400

1400

...

1800

1800

SUBARCTIC DOMAIN

SUBTROPICAL DOMAIN

1600 1400 E

1400

vII I I II SAFZ

L\\\\\l STFZ

L...Ea::... ---l --L- ....L.- ..L...- ----''--_'--__---'__---' 100

500

N

1200

60° ,----:..-~::;\li~"l'+.i"iir-lB1rr---I------m;_::.::7TV""--~[i}A:-:----r-~60°

Figure 1

Schematic diagram showing the locations of the subarctic and subtropical domains, the transition zone and the associated

subarctic frontal zone (SAFZ) and the subtropical frontal zone (STFZ). Black arrows indicate boundary current intrusions.

While the transition zone and associated subarctic

and subtropical frontal zones are large scale and semipermanent, there are mesoscale perturbations er-lbedded in them that are highly time variable. They reflect

the combined effects of intrusive boundary current instability (Holland et al. 1983; Watts 1983), weather disturbances (Frankignoul and Muller 1979; Muller and

Frankignoul 1981), Rossby waves (Kang and Magaard

1980, 1982), mixing (Rudnick and Davis 1988), and

topography, such as seamounts and seamount chains

(Roden 1987). Chief among the observed mesoscale

features are meandering fronts (Roden 1981; Van

Woert 1982; Lynn 1986), jets (Kawai 1972; Woods

1988), propagating eddies (Kitano 1975; Kirwan et al.

1978; Bernstein 1983), trapped vortices (Roden and

Taft 1985), and topographic boundary current deflections (Vastano et al. 1985; Roden 1987).

On synoptic time scales, these mesoscale perturbations often dominate the transition zone and the subarctic and subtropical frontal zones. This is seen in satellite

infrared images (Legeckis 1978), satellite altimeter

records (Cheney et al. 1983), high resolution dynamic

height sections (Roden 1977b), and direct current measurements (Schmitz 1987). The findings indicate that

the mesoscale variability in the transition zone varies

strongly with longitude in a nonuniform manner: west of

the Emperor Seamounts the amplitudes of the mesoscale perturbations are 3 to 6 times larger than east of

it. This variability can be related to the presence of the

Kuroshio extension and rugged topography in the western North Pacific basin and to the absence of strong

boundary currents and high amplitude topography in

the mid-latitudes of the eastern basin.

The above features indicate that to understand the

synoptic thermohaline structure in the transition zone,

both the large scale flow and the mesoscale perturbations need to be known. The purpose of the present

paper is to discuss the large and mesoscale aspects of

the transition zone and the accompanying subarctic

and subtropical frontal zones based on recent observations and to elucidate the salient features.

Transition and Frontal Zone Terminology __

In discussing the oceanography of the North Pacific, it

is useful to distinguish among water source domains,

transition zones, frontal zones, individual fronts, and

climatological mean fronts. A schematic vertical section

through the subarctic-subtropical transition zone is

shown in Figure 2.

The subarctic domain extends northward of about

43° N to Alaska and is characterized by a cool, low salinity surface layer, about 120 m deep in winter and 30 m

deep in summer. A well defined permanent halocline

occurs between about 120 and 180 m, which is usually
Roden: Subarctic-Subtropical Transition Zone of the North Pacific 3

I I 1 I I

SUBARCTIC SUBSASF SASF STSF STSF TROPIC DOMAIN DOMAI~ - 33.0 C4

'n 35.0

r- 33.8 33.9 ( ___SFZ ___ -

i---SFZ .. ... SALINITY TRANSITION ZONE,

1/1 I

34.0

I

~ERA/I SUMME~ I~-

45°

o

100

200

300

44° 43° 42° 41° 40°

NORTH LATITUDE

39° 38° 37° 36° 35° 34° 33° 32° 31° 30°

O,----,-----.'1r1rr-,-----,-,--rr,----,--r---.-----.---.---.-----.---,---TTT.--.,------.rr-------,

E 100

:r

fa.

200

300'----"-------'------"----'--------'-------'---__-L__---'--__---'-__---.l__----I L-__L---=====---!

100

O~~~~~~~~~

200

300 '-----'-----'-------'----...L-----'-------'--------'--_---'-__---'-__----'__----I L-__L-__L-_----1

TEMPERATURE INVERSION

!lEI STRONG WEAK 0 NONE

Figure 2

Schematic vertical section through the subarctic-subtropical zone in mid-ocean. Note the occurrence of temperature inversions

in the halocline. SASF=subarctic salinity front; SATF=subarctic temperature front; STSF=subtropical salinity front, STIF=subtropical

temperature front, TFZ=temperature frontal zone; and SFZ=salinity frontal zone.

bounded by the 33.0 and 33.8%0 isohalines. A persistent temperature inversion is often found in the permanent halocline. During summer, a distinct temperature

minimum is encountered between the bottom of the

shallow thermocline and the underlying temperature

inversion. This minimum can be traced over long distances in the central and western parts of the domain

(Uda 1963; Roden 1964). The subarctic temperaturesalinity relation (61 S) is noted for a rather constant

temperature in the salinity range from 33.0-33.8%0

(Fig. 3, left).

The subtropical domain extends southward from

31°N to about 200 N and is characterized by a warm,

saline surface layer, about 125 m deep in winter and

60 m deep in summer. The 34.6 and 35.3%0 isohalines

are usually embedded in the sharp upper halocline. In

the eastern part of the domain, a shallow salinity minimum occurs beneath the salty top layer, containing

modified subarctic and California current water (Reid

1973; Talley 1985). The subtropical 61 S relationship

has a distinctive C-shaped curve, with a well developed

intermediate depth salinity minimum of 34.0%0 near

600 m (Fig. 3, right).

The region between the subarctic and subtropical

domains is occupied by the transition zone, where temperatures and salinities gradually increase southward.

In this zone, waters of both subarctic and subtropical

origin are present in various proportions, depending

upon distance from the source regions. Near the center of the transition zone in mid-ocean, the 61 S relationship indicates an equal mixture of subarctic and

subtropical waters (Fig. 3, middle). A closer look at the

thermohaline structure reveals, however, that the

change from purely subarctic to mixed subarctic-subtropical water and from this water to purely subtropical

water is not uniform, but occurs rather abruptly near

the northern and southern limits of the transition

zone, which have the character offrontal zones.

The subarctic frontal zone in the mid-Pacific extends

from about 40° N to 43° N. The southern limit is best
,j:lo.

0"-"" ""TT'-,'-.----.------.--·.-------.----.----.·-~

83.0 e4.0 2~.O ~8.0 a'7,O

z

/')

[

/')

!.

'1:l

o

::!.

-

o

\Jl

DATI 25 JAM 18'1'4

I

LAT 2'7· o , LaNa I ~8-

:'J r 1 l ----,- -,-------,------ ~~,-~~~

32.0 ~s.o 34.0 38.0 e Ii' [I. iii i , , Iii i ,

:to 10. 10. lito. ea.

STATION 94

o

500

1000

)500

l.OIlG 1&8- DATE 02 OCT I fnl! l.AT 30-10.0 It

S r -, '-.-----'-------.----.--1 I I

32.0 33.0 S4.0 30.0 8 ., .,. ii' •• i" , •

2. ? 12. ,.,. It.

CI''----'--- ,I ii, " ". 23.0 24.0 2S.0 20.0 27.0

STAff 0,. 21 I

o

500

1000

J500

LAT 84-56.0 ,. LOMG IIUI-19.D 1 DATI: I MAR le8,

s ' If, ,

:!II.0 a'LO 311.0 3<41.0 e ' • i , Iii i

o. a 10, 10. 20.

G' ' Iiii I

84.0 RO.O 80.0 8'1.0 28.0

STATION 62

o

1:)00 I , - 'J - I

'" 500

e

.,

'";>

l/l

l/l

.,

P:: 1000 ...

20 25

22 ,

// -:; 15 -1 I 20

;"~ '7 , 15 ( '2

e

I

I

10

5 .. 7 ......... .. 5

0 2

3' 0 32.0 33.0 34.0 32.0 33.0 34.0 35.0 32.0 33.0 34.0 35.0

SALINITY SAl.INITY SALINITY

Figure 3

Characteristic vertical profiles of potential temperature, e (c), salinity, S (%0), density, a (kgm-3), and characteristic potential temperature-salinity curves in the

subarctic domain (left), in the subactic-subtropical transition zone (middle), and in the subtropical domain (right) in the central North Pacific.
Roden: Subarctic-Subtropical Transition Zone of the North Pacific 5

defined by the outcrop of the 33.8%0 isohaline, which

forms the bottom of the halocline in the subarctic domain (Fig. 2). The northern limit can be defined similarly by the outcrop of the 33.0%0 isohaline, which lies

near the top of the subarctic halocline. It is also possible to define the southern limit of the subarctic frontal zone by the vanishing of large temperature inversions (>0.25° C), which can exist only within the

subarctic halocline, for reasons of hydrostatic stability

(Roden 1964, 1977a), and by the rapid increase in the

depth of the 9° C and 10° C isotherms below the shallow seasonal thermocline.

The subtropical frontal zone in the mid-Pacific extends from about 31° N to 34° N. In winter, the northern limit is best described by a thermohaline front, in

which the 34.8%0 isohaline and the 18° C isotherm are

embedded (Roden 1980). In summer, only a salinity

front is seen at the surface. Below the shallow seasonal

thermocline, the temperature front can sometimes be

located by the rapid change in depth of the 18° C and

20° C isotherms. The southern limit of the subtropical

frontal zone can be described by the outcrop of the

35.2%0 isohaline, which lies near the top of the subtropical halocline.

One or several individual fronts occur in each of the

frontal zones. The individual fronts at any given time

are convoluted in shape, meander and shed eddies that

retain their frontal characteristics. These individual

fronts and eddies are in various stages of formation and

decay (Roden 1981) and are of prime importance in

understanding mesoscale physical processes and biological patchiness (Woods 1988).

When the individual fronts and eddies are averaged

over time and space, the irregularities disappear and a

single climatological mean front appears. In midocean, the climatological mean position of the subarctic front is near 42° N and that of the subtropical front

is near 32° N. Such climatological mean fronts are

rarely seen in synoptic and quasisynoptic surveys.

For understanding processes of transition and frontal zone formation, it is important to distinguish also

between

Confluence, which is related to the deformation field

of flow, denotes the sidewise approach of currents from

different directions and does not entail vertical velocity,

and

Convergence, which basically expresses the decrease

of current speed in the direction of flow. It can occur

in unidirectional flow and is associated with a vertical

velocity component.

While frontal zones occur 'in geographically preferred regions in response to the gyre-scale circulation

and large-scale atmospheric and topographic forcing,

individual fronts can occur anywhere in the ocean

whenever confluent or convergent flow interacts with

the ambient stratification. A clear distinction must be

made, therefore, between upper ocean frontal zones,

which are source water domain boundaries, and individual fronts, which do not always separate water masses

of different origin (Roden 1980; Fedorov 1986).

Oceanic temperature, salinity, and density fronts do

not always coexist nor penetrate to the same depth. For

this reason, it is always necessary to specifY the variable,

in order to avoid ambiguities. For example, upper

ocean temperature and salinity fronts are strong in the

subarctic region of the western and central North Pacific, yet the subarctic surface density fronts are weak or

nonexistent, because of a near balance of the horizontal temperature and salinity gradients. Likewise, in the

subtropical frontal zone, the well-defined upper ocean

salinity fronts are not accompanied by any temperature

fronts during summer, because intense solar radiation

has obliterated the wintertime horizontal temperature

gradients.

Large Scale Aspects of the Transition and

Frontal Zones _

Basic Processes of Formation

The transition and associated frontal zones are features

of the oceanic and atmospheric general circulation and

represent regions of enhanced horizontal temperature

and salinity gradients. To understand the formation of

these zones, it is necessary to investigate processes that

determine the intensities of these gradients. In the absence of molecular fluxes, horizontal temperature and

salinity gradients will intensifY when (Roden 1975)

ft.-IV' 61=-JL(\i IV' 61)_L(w ae )_iL(_1 ~) at H an n6 H an az an PG;;s Jz

a a(w'6') - an ---az> 0 (1)

and

IV' sl - (v IV' 61) iL (w as) iL (1 asz ) at -H - - an ns -H - an az - an p az

a a(w's') - an az > 0 (2)

where the bars represent the mean values and the

primes the turbulent fluctuations, IV' 61 and IV' S I the Ii. Ii •

magnitudes of the horizontal temperature and salinity

gradients, p is density, cus is the specific heat at constant

volume and salinity, t is time, z is depth, vnO is the horizontal velocity component normal to the temperature

front, Vns is the horizontal velocity component normal
6 NOAA Technical Report #105

to the salinity front (the normals n are taken in the

direction of increasing values of eand S), q, is the vertical component of net radiative heat flux,s, is the vertical component of freshwater (precipitation) flux, and

<w'e'> and <w'S'> are the turbulent heat and salt fluxes.

The dynamics of transition and frontal zone formation are contained in the four right hand terms of

Equations 1 and 2. They depend upon the joint effects

of velocity shear, thermohaline stratification, and geographically varying heat and salt fluxes. The relevant

physical processes can be summarized as follows:

1) Differential horizontal advection of temperature

and salinity. These processes will increase the intensity

of horizontal gradients in regions of current

confluence and convergence.

2) Differential vertical advection of temperature and

salinity. These processes will increase the intensity of

horizontal gradients in regions where large horizontal

shear of the vertical velocity component occurs in the

thermocline or halocline. Differential Ekman pumping

atop the thermocline is an example.

3) Differential radiative heat and freshwater fluxes.

These processes will increase the intensity of horizontal

gradients in the vicinity of large scale cloudines~ and

precipitation fronts and are important only in the upper layer of the ocean. The doldrum salinity fronts are

an example (Roden 1975).

4) Differential turbulent heat and salt fluxes. These

processes act on both large and small scales increasing

the intensity of horizontal gradients near climatic transition zones which, in turn, lead to buoyancy flux and

double diffusion fronts (Rostov and Zhabin 1987).

Terms 1), 2), and 4) involve the configuration of the

wind field. In regions away from boundary currents, the

wind field configuration often plays the dominant role,

where frontal zones are formed (Roden 1980, 1981;

Camerlengo 1982). Equations 1 and 2 imply that the

intensification of horizontal gradients depends upon

the sum of the four terms being positive; it is not necessary that each individual term is positive.

Horizontal density gradients are linked to horizontal

temperature and salinity gradients by the equation of

state for seawater (Gill 1982). to first approximation,

the horizontal density gradients will intensity when

(3)

120E 140 160 180 160 140

60N

50

40

30

20

+=---=,3 10

120 W

Figure 4

Surface currents in the North Pacific during winter. Sc,lid black lines indicate clearly defined current boundaries, dashed lines

less clearly defined ones. After Schott (1943).
Roden: Subarctic-Subtropical Transition Zone of the North Pacific 7

The coefficient varies from -1 - 10-40 C-Jat 0° C

-4 -I . 1

to -3 - 10 ° C at 26° C, while p as varies from

7.8 - 10-4 (%0)-1 at 0° C to 7.3-10-'1 (%0)-1 at 26' C under oceanic conditions. Typical cross-frontal temperature and salinity differences vary between 2 and 10° C

and 0.3-1.2%0, respectively. Because the thermodynamic coefficients are of opposite signs, while large

scale horizontal temperature and salinity gradients in

the transition and frontal zones are of the same sign

(both e and S increase southward), the two right-hand

terms in Equation 3 oppose each other. When these

two terms are equal, the horizontal density gradients

vanish. This is often observed in the subarctic frontal

zone, where the intense temperature and salinity fronts

in the upper layer are not accompanied by any density

fronts (Roden 1972;Joyce 1987).

The above discussion indicates that a variety of physical processes are responsible for the formation of transition and frontal zones and that no single process can

account for all of the observed features. Nevertheless,

in specific regions or the orth Pacific, it is possible to

identify the dominant processes controlling the formation of these zones and to relate many of the observed

features to them.

Relation to Subarctic and Subtropical Gyre

Confluence

The large scale ocean circulation of the mid-latitude

Pacific is characterized by a cyclonic subarctic gyre and

an anticyclonic subtropical gyre, as indicated in Figure

4, based on Schott (1943). Two current confluence

lines stand out, one in the transition between the sub30'

160'

160'

T. ·C

100m

SUMMER 1966

~~~~---l-J40'N

130'

1600

160'

130·

S, °/00

·~I/P.;:o-tc-~~4----+--.e:::.~1-----100 m 30'

SUMMER 1966

160 1400 1500E 0 1300

Figure 5

Subarctic-subtropical transition zone in the western Pacific. Note the occurrence of two sharp fronts at the boundaries of this

zone. AfterJapan Oceanographic Data Center (1968. 1969).
8 NOAA Technical Report #105

ATMOSPHERE FORCING OCEAN (EKMAN) RESPONSE

50· SUBARCTIC t+ H. DIVERGENCE (UPWELLING) ZONE

WESTWIND ..

40·

H N SUBARCTIC •• H. CONVERGENCE (FRONTAL) ZONE w

0 ·.... l-

:;) ·.... ·.... TRANSITION ZONE l-

<[ ·.... --l 30· ·....

SUBTROPICAL+ • +i CONVERGENCE (FRONTAL) ZONE

• EASTWIND ttttt 20·

TROPICAL +++H DIVERGENCE (UPWELLING) ZONE

·....

Figure 6

Schematic diagram showing the main wind belts in the central l\'orth Pacific and the upper layer Ekman response to them.

Note the Ekman transport convergence zones to the right of the planetary west and eastwind maxima.

arctic and subtropical gyres, centered near 42° , and

the other within the center of the subtropical gyre near

23° . It is tempting to associate these current

confluences with thermohaline transition and frontal

zones. This does not always lead to correct interpretations, however, because frontal zone formation does

not depend upon confluence alone, but upon both the

intensity of the initial thermohaline gradients and a

favorable flow field configuration (which includes both

confluence and convergence). The confluence between the subarctic and subtropical gyres leads to a

transition zone at the boundaries of which two thermohaline frontal zones are located (Fig. 5), owing to convergence of flow within the Kuroshio and Oyashio currents. On the other hand, the confluence within the

subtropical gyre is not associated with any thermohaline transition zone, because the upper ocean temperature and salinity differences of the meeting currents

(North Equatorial Current and Kuroshio recirculation)

are small.

Relation to the Configuration of the Surface

Ekman Transport Field _

The planetary-scale atmospheric circulation in the

North Pacific is characterized by westerlies in the north

and easterly tradewinds in the south, separated by a

zone of weak and variable winds, as shown schematically in Figure 6. The net effect of these winds is to

move the surface layer of the ocean at right angles to

the wind direction. This leads to a general confluence

of Ekman transports and to the formation of two convergence zones (where the decrease in transport in the

flow direction reaches a maximum), one on the

equatorward side of the westwind peak and one on the

poleward side of the eastwind peak. If the initial horizontal thermohaline gradients were zonal and uniform,

the surface transition zone would coincide with the

zone of Ekman transport confluence and the surface

subarctic and subtropical frontal zones would form

where the Ekman transports converge most strongly.

These idealized conditions are observed at times in

the central North Pacific, where the baroclinic flow is

weak and wind forcing dominates. In Figure 7 are

shown the Ekman transports for April 1968, computed

from the stress of the geostrophic wind on a 1° by 1°

latitude-longitude grid (Roden 1970). There is a general confluence of Ekman transports between about

31° Nand 43° N. Southward transports decrease most

rapidly between 43° and 40° N and northward transports decrease most sharply between 31° Nand 34° N,

forming two convergence zones. A concurrent vertical

salinity section taken along longitude 158° W in April

1968 is shown in Figure 8. It is seen that the low salinity

subarctic water terminates with a front at 39° Nand

that the high salinity subtropical water effectively terminates at the front near 34° N. Several secondary fronts

occur in conjunction with these water mass limits, showing the existence of a subarctic frontal zone between

39° and 45° and a subtropical frontal zone between 31 ° Nand 34° N. The location of these frontal

zones agrees closely with the location of the Ekman

transport convergence zones, in this case.
Roden: Subarctic-Subtropical Transition Zone of the North Pacific 9

50°

APRIL 1971

170 0 E

)f.

,

.

140°

EKMAN TRANSPORT

• <4.104 g em-I sec-I :

t 4-8

f >8 . 4.-'" . \. \. \.

..... ;/~~~ ~~ \.\.\.\\\\\\\\\\\\.\.\.

. . . /1 1//./.// . . . '..'..'-.'-.'-. '-. \ \ 1 1 1 \ \ \ \. \. \. \ \ \ \ \ \ \ \ \ \ \ \. \.\.

111-1"' ,','. elf 111/ / / / I 1 1\ \ '..'..'-.'-.'-.'.. \ \ 111\ \ \ \ \. \. \ \ \ \ \ \ \ \ \ \ \ \ \.'" 11 ... ;", .'. :..•...;..' 1111j II, 1 1 \ \ \ \. \.'..'-.'-.'.. \. \ \ 1\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ .. '~6DATE .. 11 // , J 1\ \ \ \ \. \.'..'..'..\. \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \

'111111\\\\\\.\.'..'..'..\\\\\\\\\\\\\\\\\\\\\\\\

40° ·1111 \\\\\\\.\.'..'..'..\.\\\\\\\\\\.\.\.\\\\\\\\\\\ 40°

'111\\\\\\\.\.'..'..'..\.\.\.\\\\\\\\.\.\.\.\.\\\\\\\\\ 11\· .\.\.\.\\\\\.\.\.\.'..\.\.\\\\\\\\\

.\.\.\.\.\.\.'..'..'..\.\.\.\.\\\\\\

.\.\.'..'..'..'..'..'..'..'..\.\.\.\\.\\

.'-.'-.'-..

30°

20°

N

. 1

.\\\ ·lii

\\\\\\\\\.

\\\\\\\\\\ ..

\ \ \ \ \ \ \ \ \ \ \ "\ "\ \ \ \ \ \ \ \ \ \ "\ i \ i i . . \ \ \ "\ \ \ \ \ \ \ \ i i } } i

j

i

1lll1l11111 11l 1 , , , , , t

TTTT . . MINAMI TORI

SHIMA·

II

· r t t WAyt "\ "\ · . . . . . . . . . . . . . . r r r T rorlp \ \ \\ \

· . . . . . . . . . 1 TTTr II ITT 1 11 1"\ \ \ r r r 111111 111Ttl\ 1 1, , , , ,

rTf ffff ff f T ; i

f.f r! Ii ttt " , , 200

.WAKE, " " , nnn N

III ii iii iii iii i

i i

170 0 E

Figure 7

Ekman transport (gmc's-I) computed from the stress of the geostrophic wind on a 1° by 1° latitude-longitude grid. After

Roden (1970).

Such good agreement as discussed above cannot be

expected in regions of strong and dominating

baroclinic flow, such as near the Kuroshio-Oyashio

confluence, or in regions influenced by the Kuroshio

extension and other boundary currents. Even in regions of weak baroclinic flow, the relationship between

Ekman flow patterns and frontal zones is not simple,

because it depends upon the existing thermohaline

stratification and the duration of and intensity of atmospheric forcing. Firstly, a given favorable configuration

of the Ekman transport field will lead to the formation

ofstrong frontal zones in some geographic regions, but

only weak ones in others, depending upon the intensity

of the initial horizontal thermohaline gradients. Secondly, a favorable configuration of such a field must

persist over a certain length of time before measurable

frontal zone formation occurs. Thirdly, frontal zones

do not disappear instantly after wind forcing ceases, but

take time to decay. The exact formation and dissipation

times offrontal zones are not known.

Relation to the Configuration of the

Wind-induced Vertical Velocity Field _

The large scale winds in the subarctic are predominantly cyclonic and those in the subtropics are pre-
<:

o

...

0

z

0

::1 !11J~~)~~l7~C-/- ~~(~~n.~ ~~\S?~~ I

e:..

II)

"0

0

...

...

=II:

...

0

'"

810

720

855

900

495

765

450

360

405

315

::1 540 i'!

!!! 585

;': 630

<L

675

945

990

1035

<::>

1080

1125

1170

1215

34.4'

('-...

Figure 8

Meridional salinity section along 158° W in April 1968. Note the occurrence of multiple salinity fronts in the subarctic frontal zone (39°-45° N) and the subtropical

frontal zone (31°-34° N). After Roden (1970).
Roden: Subarctic-Subtropical Transition Zone of the North Pacific 11

10

-10

158E

50N

130W 23N

Figure 9

Three-dimensional plot of the wind induced vertical velocity at the bottom of the upper mixed

layer. Note the sharp slope of the contours between latitudes 42° Nand 48° N.

dominantly anticyclonic This induces upwelling in the

former and downwelling in the latter region, while in

the transition zone there are enhanced horizontal gradients of the vertical velocity component. The differential vertical motion leads to differential vertical advection of heat and salt, which is an important process in

transition zone formation at depths below the upper

mixed layer. A three-dimensional plot of the 30-year

mean wind induced vertical velocity in winter is shown

in Figure 9, on a 1° by JO latitude-longitude grid. The

vertical velocity is derived from the curl of the stress of

the quasigeostrophic wind (geostrophic wind with a

small frictional term included to allow for cross-isobar

flow of about 15°). Clearly, the geographical variability

of the vertical velocity component is large and

nonuniform. Upwelling predominates north of 40° N

and downwelling south of it. It is noteworthy that the

strongest horizontal gradients of the vertical velocity

occur between 42° Nand 48° N, and not where W = O.

For this reason, the expected position of the subsurface

transition zone is in the same latitude belt and not near

the line where the curl of the wind stress vanishes. This

is borne out in the meridional temperature section

along 155° W (Fig. 10), where the largest horizontal

thermal gradients occur between 42° Nand 48° N below the upper mixed layer.

Mesoscale Aspects of the

Transition and Frontal Zones _

On the planetary scale, the subarctic-subtropical transition and associated frontal zones are semipermanent

features of the general circulation, slightly fluctuating

about their mean positions. On the mesoscale, the

dominating features in these zones are individual

fronts, eddies, and jetlike flows that are highly variable

in both time and space. In addition, there are

quasiregular long wave disturbances (Roden 1977b;

Kang and Magaard 1980; White 1982) and topographically induced perturbations (Roden 1987). These mesoscale features are important in understanding the

synoptic variability within the transition and frontal

zones. In the following an overview will be given of the

salient observed mesoscale structures.

Mesoscale variability of

dynamic height and baroclinic flow _

The mid-latitude North Pacific can be divided into two

basins, separated from each other by the Emperor Seamount Chain that extends in a nearly north-south direction in the vicinity of 170° E for a distance of

2500 km. In the western basin, the dynamic height

perturbations are three to six times larger than in the

eastern one (Fig. 11). Maximum perturbations in the

western basin reach 8 J kg-1

, equivalent to about 0.8 m

in sea surface elevation. The large variability of sea

surface height in the western Pacific is seen also in

records derived from satellite altimetry (Cheney et al.

1983). The reason for the large perturbations in this

region is generally attributed to the instabilities and

eddy shedding of the Kuroshio extension, which meanders eastward along this longitude (Kawai 1972; Roden

et al. 1982).
-

z

0

i;3

n

5"::;.

e:.

"C

0

::l.

100 I 0-

I

\>1

I 200

JAN. 1966

STA.I II 12 13

100 1-==//=, i""i-5--:;;:'R.;'7T?~)= I J 0:;/'=' "- J / ) "I 7' :=;;.;;' J J A I I

200 f-==='- .......... JI7'r Ie,..., / 7 '\ IT\: J 1\ 71\/V J- C ( I C / 7 I 1 7 7 C I \ \ I

300 1...... .<:"( "--""""7""-"":/ )./ \./ 7{ \ ....../ / ,J / l,e 7£ 11300

400 b-/ ..........." __ ::::;;0-'" ~L"""-/ :;........L \..... 7 1 7/'·....._.;;/ JL 7"\ / I l I 400

500~" J~ ..J --------------= { 155 0

:7, 7 7 I W I 500

600 I /..c "'--=C : I 600

LAT.26' 27" 28' 29' 30' 31' 32' 33' 34' 35' 36' 37' 38' 39' 40' 41' 42' 43' 44' 45' 46' 47' 48' 49' 50' 51' 52' 53' 54' 55' 56'N

Figure 10

Meridional temperature section along 1550 W in January 1966 taken during the Zetes Expedition. (Scripps Institution of Oceanography, 1970)
Roden: Subarctic-Subtropical Transition Zone of the North Pacific 13

LONGITUDE AT LATITUDE 35°N

140° 1500 1600 1700 E 180· 170·W 160· 150· 1400 130· 120·

221--.....1...--'----'-----'---'--.........---"----'---.1.----1

20 0/1000 db

DEGREES NORTH LATITUDE

4;:-3__--,4r-1__.;39r-_--;37:....-_-=;35:.___3~3:.--__=;.3:....1_--.:2;:9:....-_--=2=r7__--=2=r5:....-_--=2=r3:....-_--=2;.:,1

0L...-_--J1L...-_--J1L...-_....J1__....J1__---l1__---l1__--'-1__..J..1__..I....-1__..I....-1__l.J1

Figure 12

Relative dynamic height across the

transition zone in the western

North Pacific at various pressures

(depths). MterRoden (1979).

-

-

-

-

............ -

o

Figure 11

Relative dynamic heigh t between Japan and California

in a section along 35° N near the center for the subarctic-subtropical transition zone. Note the large amplitude perturbations west of the Emperor Seamounts.

Based on Kenyon (1978) and Roden et aI. (1982).

0/1500 db o •

.

the Kuroshio extension (here located near 33° N); the

trough to crest equivalent sea surface elevation differences are of the order of 0.4 m throughout the region.

This implies that there are sources other than the

Kuroshio extension that contribute to the sea suTface

height variability. A possible source is the subarctic current which occurs in the vicinity of the subarctic front;

it, too, is meandering and eddy shedding (Kawai 1972).

The transition zone of the western Pacific thus appears

to receive eddy injections from both the north and the

• • •••• 300/1500 db - .

o 0

o

150/1500 db .0··0 •• 0 •••• -

...... .. . . . .. . .- o • • ...

- 450/1500 db ...................•.

o

o

00

CALIFORNIA

o 00

o

... ..

o ... .. o

o

... ...

o

o

o 0

o

00·

• 0

.

.. .. 0.0

.0. o. 0 ·0 o. 600/1500 db ...... .... .......•....•..•.•..•:

o

o ..

..

o· ...

EMPEROR SEA MOUNTS

.... o 0

.... • 0

• 0

o

o o

18

12

14

• 1200/1500 db 2ke. ••••• • •••••••••••••••••••••••••••••••••••••• - ..

4-

24 I1680 E

APR 1971

22 I20 I00

181- 0 0

0

0 00 161- 00

...,

" 16

14~.· • • •••

C) ••• • •••• GJ •• • • I 12f- •• •• •• .. ....

Ul

0 I-'

JAPAN

1000

2000"-----------""------- --L.:>J

o

o 10 r- • .-. •••••• • ••••• ..... .. .. ..... « ••- •• z 0

>-0 8.-..- ...•.. .•.....•. . .... 6 -! ••

Western Basin-Mesoscale dynamic height perturbations are apparent not only in zonal sections, but occur

in meridional ones as well. In Figure 12 are shown the

relative dynamic heights across the transition zone in

the western basin at 168° E, about 250 km west of the

Emperor Seamounts. Two features stand out: the perturbations between 25° Nand 43° N are vertically coherent and they have dominant wavelengths of about

400 km (Roden 1979). There is no sign that the amplitudes of the perturbations decrease with distance from
14 NOAA Technical Report #105

BCV (CM/SEC) AT 1680

E, APR 1971

z z z z z z z z z z z z z z z z z z z z z

'" :; !;i! '" CD r- ID U1 ... '" '" ;;; 0

'" CD r- ID U1 ... '" '" N 0

... '" '" '" '" '" '" '" '" '" '" '" '" '" '" '" N '" '" I I , I , ,

0

200

20

(f) 400 cr::

a:

(D

.......

u 600 w

0

z 800

w

cr::

(f) 1000 (f)

w

cr::

(L 1200 0 0

1400 I \ 11~m \

1600

Figure 13

Mesoscale variability of the zonal component of baroclinic flow across the subarctic-subtropical transition zone in the western

Pacific at 1680 E, after the end of the 1971 winter. Speeds are in cm S-I. After Roden (1984).

south, which may account for the semiconstant perturbation amplitudes throughout the zone.

Associated with the dynamic height perturbations are

those of baroclinic flow. The latitudinal variation of the

zonal flow component along 1680 E is shown in Figure

13; the 2442-km long section was taken between 3 and

11 April 1971 after the end of the winter seasor and

has a 37-km station spacing and a 3-m vertical sampling

interval (Roden 1972; Roden and Fredericks 1987a).

The pattern consists of alternating eastward (positive)

and westward (negative) flows, spaced at quasiregular

intervals of a few hundred kilometers. Maxi:num

speeds are of the order of 0.5 ms-I and the depth penetration of the flow, as measured by the 0.02 ms- 1 isotach, is about 1000 m. There is strong vertical coherence of the flow. The subarctic current is centered near

41 °40' N and the Kuroshio is near 33° N, with surface

baroclinic speeds of 0.38 ms- 1 and 0.48 ms-I, respectively.

How representative are the above findings taken

shortly after the end of the winter season in 1971? Indications are that the pattern of alternating flows persist,

while the location of the cores of the subarctic and

Kuroshio currents shift with time. In Figure 14 is shown

the latitudinal variation of the zonal flow component

along 167° E during the summer of 1982 (Roden and

Taft 1984) using the same 37-km horizontal and 3-m

vertical sampling intervals. The subarctic current is

centered near 41 ° N and has a surface speed of about

0.26 ms-1 (Roden 1984). The Kuroshio is encountered

near 35°30' N, with a surface speed of 0.54 ms-1 (Roden

and Taft 1985). In November 1983, using a coarser

horizontal sampling grid along 1650E, Joyce (1987)

found the subarctic current to be located near 41° N

with a surface speed of 0.2 ms- 1 and the Kuroshio near

34°30' N, with a speed of 0.85 ms-I. He also found the

0.02 ms-1 isotach to penetrate to 1000 m under the

subarctic current and to about 2500 m under the

Kuroshio Current.

Direct current measurements at an array placed

along 165° E (Schmitz 1987) and 167° E (Hamann and

Taft 1987) confirm the strong vertical coherence suggested by the baroclinic currents computed from the

distribution of mass. Below 2000 m, however, the

barotropic flow components dominate and near the

bottom they exceed the baroclinic components by an

order to two orders of magnitude. At 41 ° N, the abyssal

flow is surprisingly stable and almost unidirectional, for

reasons unknown (Schmitz 1987).

Eastern Basin-In contrast to the large mesoscale perturbations observed in the western Pacific, those en-
Roden: Subarctic-Subtropical Transition Zone of the North Pacific 15

Bey (eMlS) AT 167°E JUL 1982

41" 40" 39" 38" 37" 36" 35" 34" 33" I I I I I I , I

100

200

-40

300

400 -30

500 -20 (J)

0::

a: 600 III

u

w

0 700

::: -to w 800 0::

::J

(J)

(J)

w 900 0::

lL

1000

1100 I 100 KM I

1200 0 0 0 0 0 0

1300

w( ) f~ \ W) i

w E E E E W

1400

1500

Figure 14

Mesoscale variability of the zonal component of baroclinic flow across the subarctic-subtropical transition zone in the western

Pacific at 167° E, in the summer of 1982. Speeds are in cm S-I. After Roden (1979).

countered in the mid-latitude eastern Pacific are small.

The main distinguishing characteristics of the eastern

basin are the absence of strong boundary currents that

penetrate into the ocean interior and the smooth bottom topography between 30° Nand 500 N. Thus, two

major sources of eddy variability are lacking in the region. Nevertheless, a low amplitude mesoscale perturbation field is common in the mid-latitudes, dominating the flow on synoptic time scales.

A representative dynamic height section extending

from the subarctic domain across the transition zone

into the SUbtropical domain is shown in Figure 15. The

2781-km long section along 1580 W is based on 27-km

station spacing and was occupied between 27 September

and 8 October 1975 (Roden 1977b; Roden and

Fredericks 1987b). The noteworthy features are the

northward decrease of the perturbation amplitudes and

their rapid attenuation with depth, particularly north of

340 N (the southern limit of the transition zone). The perturbation heights vary from 2.5 J kg-1 near Oahu to less

than 1.0 J kg-I in the subarctic domain (equivalent to

0.25 m and less than 0.1 m of sea-surface elevation, respectively). Similar results were obtained by Talley and

DeSzoeke (1986) when analyzing a high re-solution dynamic height section along 1520 W in May 1984.

The temporal variation of the zonal baroclinic flow

component along 1580 W is shown in Figures 16 and 17

for spring and fall based on data by Roden and

Fredericks (1987, b and c). In both seasons the basic

current structure is similar. Throughout the transition

zone and in the subarctic domain, the mean flow is

eastward with a sluggish speed of 0.02- 0.03 ms-I. The

amplitude of the superposed eastward and westward

perturbations rarely exceeds 0.1 ms-I. The depth penetration of the flow, as measured by the 0.02 ms-I isotach, is about 600 m. In the subtropical domain, the

character of the flow changes. The mesoscale perturbations extend deeper (to 1000 m) and the perturbation

speeds become stronger (0.2-0.4 ms-I), especially near

the Hawaiian Ridge. The amplification of the mesoscale perturbations and their banded structure near

this ridge is believed to result from topography-flow

interaction (Mysak and Magaard 1983; Oh and

Magaard 1984; Sun et al. 1988).
16 NOAA Technical Report #105

6

3

1200/1500 db 2

DEGREES NORTH LATITUDE

47 45 43 41 39 37 35 33 31 29 27 25 23 24 I I 1

gradient. when'*< _pgc2 = -4.5.10-3 kgm-"

approximately.

North Pacific Low Stability Gap-The meridional

distribution of N 2 from the subarctic domain across

the transition zone into the subtropical domain is

shown in Figure 18. The section is along 1580 Wand

illustrates conditions in early spring and early fall,

based on high resolution sampling (Roden and

Fredericks 1987, band c). Only frequencies larger than

0.01 rad S-I have been contoured to bring out the main

points. The outstanding feature is the seasonal low stability gap between 430 Nand 370 N, which has important consequences for the depth penetration of wind

mixing and thermohaline convection.

In winter and early spring, wind mixing and convection in the subarctic domain to the north of 430 N is

limited to the upper 150 m by a hydrostatic stability

maximum related to the permanent subarctic halocline. To the south of 370 ,a developing shallow

spring thermocline limits these processes to the upper

50 m. In the low stability gap, where the subarctic halocline is absent and spring warming has not progressed

far enough north, wind mixing and thermohaline convection can penetrate deeper than elsewhere in the region (see Fig. 28 below).

As the season advances, the low stability gap narrows

until it completely disappears in summer and early fall.

At this time an intense, shallow, high-stability layer related to solar heating covers the entire transition zone,

restricting wind mixing and convection effects to the

upper 30-50 m. As winter approaches, cooling destroys

the upper high stability layer in the northern part of

the transition zone and the gap opens again.

Three-Dimensional Variability of the ViisiHi Frequency-The three-dimensional variability of the hydrostatic stability is shown in Figure 19 in a series of

meridional sections through the subarctic domain and

adjacent subarctic frontal zone. The sections cover the

region between 1500 Wand 1580 Wand were obtained

in the early fall of 1975 (Roden 1977a). There are two

layers of high stability separated from each other by a

70-m thick layer of minimal stability. The upper highstability layer occurs between 30 and 50 m and increases

in strength southward; there is little variation with

longitude. The lower high-stability layer is encountered

between 120 and 150 m. Its southward limit depends

strongly upon longitude, varying from 450 N at 1580 W

to 41 0 N at 1500 W. On the other hand, the intensity of

this layer does not vary much with geographical position.

Mesoscale Variability of the Thermohaline Structure

in the Upper Ocean-The mesoscale thermohaline

structure in the mid-latitude North Pacific is complex.

The complexity arises from the interaction of currents

of different origin among themselves as well as with the

(4)

.'

'- .'0: .....

..' ......

......... .'. ...... .....

..... •••••• 0° •••: •••:.- ••••

.....

..'

2 (1 g) N =-g paz + C2

..

: 0° ••••

0" •••••••

..

..... 300/1500db ••••

: : ..

.......

..'

O/1500db.'

150/1500db.0" ••••: ••••••••••

158°W

OCT 1975

..' ......

.0° •••••

............. ,,:.:"0°

5 900/1500db 4 _- .

..,

15

II

9

14

10~"'"

16

21

19

'.'

121'-"

18

17

22

23

20

C'

13 -,

where g is the acceleration of gravity and c is the speed

of sound. In the pycnocline, the first right-hand term

dominates; elsewhere, the two right-hand terms 2.re of

the same order of magnitude, but of opposite sign. Stability exists when N 2 > 0 or, in terms of the vertical

density

Figure 15

Relative dynamic height in the transition zone in the central

North Pacific at various pressures (depths). After Roden

(1977b).

Mesoscale Variability of Hydrostatic Stability in the

Upper Ocean-Hydrostatic stability is a fundamental

oceanic parameter affecting the depth of wind mixing,

thermohaline convection, and potential vorticity dynamics (Rhines 1986). It can be expressed by the

Viiisiilii stability frequency

8 600/1500 db

................................................................ 7 ..... ••••••••••••••••• ••••

........
Roden: Subarctic-Subtropical Transition Zone of the North Pacific 17

BCV [CM/SEC) AT 158°W, APR 1968

z z z z z z z z z

z z z z z z

'" .., z 0 '" '" N '" CD r- w

'" CD W '" .., '" '" .,; '" '" '" '" '" '" '" '" '" .., ;; !; .., '" '" '" '" '" '" '" I I I I

I I ,

0

I~ 6

1

V 200

(f) 400 0:::

a:

CD

u 600 w

0

z 800

w

0:::

::J 1000 0 0 (f) \ (f) 0 0

)

w 0

) 0:::

CL 1200 L-J

100 km

1400

1600

Figure 16

Mesoscale variability of the zonal component of baroclinic flow across the subarctic-subtropical transition zone in the eastern

Pacific at 1580 W, in the spring of 1968. Speeds are in cm S-I. The station spacing is 54 km.

BCV (CM/SEC) RT l58°W, OCT \975

z z Z Z z r- z z z z r- z z z z z r- z

w '" 0

'" '" w on ;;; '" on fj gj ;:; '" '" '" '" N N N

I , , I

0

200

If) 400 D:::

IT

CD

u 600 w

0

z 800

w

D:::

If)

::::l 1000 (J)

W

D:::

(L 1200

1400

1600

Figure 17

Mesoscale variability of the zonal component of baroclinic flow across the subarctic-subtropical transition zone in the eastern

Pacific at 1580 W, in the early fall of 1975. Speeds are in cm s-'. The station spacing is 27 km.
18 NOAA Technical Report #105

atmosphere above and the bottom topography below.

This produces a great variety of observable thermohaline structures ranging from isolated eddies and convoluted fronts to more organized eddy and wave-like perturbations, all of which change with time. It is, in

general, quite difficult to obtain representative synoptic

thermohaline structures from a slow moving ship

(3-5 ms-I). Only those mesoscale structures that move

and evolve slowly compared to the ship's speed can be

described and understood adequately. Fast evolving

structures are better studied by remote sensing, provided they have a clear surface signature (Stewart

1985).

Basic Mid-Latitude Frontal Structures-Three main

frontal zones occur in the mid-latitude North Pacific:

the subarctic frontal zone, the frontal zone associated

with the Kuroshio extension (western basin only) and

the subtropical frontal zone. The thermohaline fronts

occurring in these zones and the thermohaline eddies

encountered in them have distinguishing characteristics, as shown in Figure 20. The subarctic front (top) has

well-defined horizontal temperature and salinity gradients in the upper layer which balance each other in

such a way that the resulting horizontal density gradients are small. The Kuroshio front (middle) has strong

temperature, salinity, and density signatures from the

surface to great depths. The subtropical front is primarily a salinity front in the upper layer, with only weak

horizontal temperature and density gradients.

Interannual Variability of Thermohaline FrontsOnly a few repeat hydrographic sections exist that permit one to illustrate the mesoscale interannual variability of both temperature and salinity fronts in the open

ocean. In Figures 21 to 24 are shown the frontal

changes over a six-year period (1981-1986) in the western and eastern North Pacific basins. The sections

along 170° E and 175°30' E cover the latitude belt between 38° Nand 48° N and were occupied regularly by

the Japanese RV Hokusei Maru during the third and

fourth weeks ofJuly (Hokkaido University, 1982-1987).

In the western basin at 170° E the outstanding features are the constancy of the position of the subarctic

temperature and salinity fronts. Over the six-year period, these fronts occurred within 50 km of latitude

43° N. The thermal fronts are strongest between the

bottom of the seasonal thermocline and 600 m and the

salinity fronts between the sea surface and 300 m. The

intensity of these thermohaline fronts varies from year

to year, but the most notable changes occur between

1985 and 1986. The temperature section illustrates also

the interannual variability of the intensity of the shallow

temperature minimum: while it is always present north

of the subarctic front between about 80 and 180 m,

temperatures of less than 2° C were encountered in

only two of the six years.

In the eastern basin at 175°30' E, the intensity of the

thermohaline fronts is not only weaker, but the

interannual variability of the position of these fronts is

larger, varying between about 45° N in 1981 and about

13° N in 1986, when judged by the positions of the 4° C

isotherms and the 33.8%0 isohalines. The shallow temperature minimum to the north of the front also is

more time variable: well-developed in 1985 and 1986, it

was not encountered at all in 1982 in the latitude range

investigated.

DEGREES NORTH LATITUDE

047 46 45 44 43 42 41 40 39 38 37 36 35 34 33 32 31 30 29 28

Figure 18

Mesoscale variability of the ViiisiiHi stability frequency across the subarctic-subtropical transition zone in early spring

and early fall. Units are in rad S-I. Shading has been added for emphasis. Note

the low stability gap in early spring.

158°W

OCT 1975

50

250

200

100

150 = 200 158°W

APR 1968 250

E 300 .

J:

....

0- 0 W

0
Roden: Subarctic-Subtropical Transition Zone of the North Pacific 19

Figure 19

Mesoscale variability of the Viiisiilii stability frequency in the

subarctic domain of the central North Pacific. Units are in

rad S-I. Shading has been added for emphasis. After Roden

(l977a).

200 L-_L-_.L-_..l-_...l.-_--'-_--'-_~_....J

E

r

fa..

w

o

DEGREES NORTH LATITUDE

047 46 45 44 43 42 41 40

<CO.OI

~I~·W 14 -18 SEP 1975

39

The above examples indicate that the interannual

variability of the subarctic front depends upon longitude and that findings from location cannot be extrapolated to other nearby locations necessarily.

The interannual variability of the Kuroshio was investigated by Mizuno and White (1983). The authors contoured the temperature field at 300 m, obtained by

expendable bathythermographs (XBT) dropped from

ships-of-opportunity. They found that the variability was

dominated by large amplitude, quasistationary meanders, confined to the latitude interval between 32° and

38° N, indicating Kuroshio excursions of about 300 km

from its mean position near 35° N.

Eddy and Wavelike Perturbations in the Western

Basin-The western North Pacific is dominated by the

meandering and eddy-shedding Kuroshio and Oyashio

currents and their seaward extensions. In the sea area

off northeastern Japan, where branches of the cold and

low salinity Oyashio intermingle with offshoots from

the warm and saline Kuroshio, numerous fronts and

frontal eddies are found (Kitano 1975; Roden 1975;

Vastano and Bernstein 1984). Figure 25 shows the thermohaline structure in the upper 300 m. In the 400-km

long section, taken along 40° N in April 1971 (Roden

and Fredericks 1987a), six thermohaline fronts spaced

about 60-70 km apart are encountered. Temperature

gradients of up to 9° e/30 km and salinity gradients

exceeding 1%0/30 km occur in this region. There are

numerous temperature inversions, mostly in the cold

Oyashio water mass.

Large mesoscale thermohaline perturbations occur

not only in the vicinity ofJapan, but also in the interior

of the western basin. Typical temperature, salinity, and

density perturbations across the transition zone at

168° E are shown in Figures 26-28. The sections are

based on a 37-km station spacing and were occupied in

early April 1971. Several interesting features stand out.

The prominent feature near 42° N is the subarctic

front, which has deep, vertically coherent signatures.

Just south of it, between 40° Nand 42° N, there is an

area of very uniform temperatures, salinities, and densities between about 30 and 300 m. This is the low

stability gap, where thermohaline convection and wind

mixing can penetrate deeper than elsewhere in the region during winter. The convection is initiated by the

cooling of high salinity water of Kuroshio origin; presumably, this is an episodic event occurring during outbreaks of arctic air.

Beneath the upper atmospherically influenced layer,

large amplitude quasiregularly spaced oscillations are

encountered, which are indicative of wavelike differential vertical motion or an organized eddy field of preferred wavelengths. A spectral analysis of these oscillations (Roden 1979) showed a well-defined peak at a

wavelength of 400 km and most of the spectral energy

concentrated between the wavelengths of 300 and
20 NOAA Technical Report #105

~Et1P~flTUR~ ICJ.~t ~E

; Ii]

'0

60

90

120

ISO

lBO

210

240

270

'DO

30

60

90

120

ISO

180

210

240

270

300

5 678 'g

-II

10

z b z

1', $

;;

--, 265 263

w SIGf1A-T (MG/CUBIC eM)

-55 - '2 :z: -.:...i1i_----'&~--"'~_

JEr'FERATUREICI Af 1520 w SRLIN!IY I PER MILLEI SIGMR-r IMG/CUBIC Cf'1)

2: I\ I I f353 25 8'''I I so

•

7'

10 100

125

150

="'

175

200 56

: f'J'L~ A\r"-" 225

250 rJ'2262 258 I ~4

~. r'. '\. 26.1 60

Z75 34 3 275

300 300 26.3

Figure 20

Main types of upper ocean temperature, salinity, and density fronts in the mid-latitude North Pacific. (Top)

Subarctic front in early April 1971; (middle) Kuroshio front in April 1971; (bottom) subtropical front in

February 1974.

600 km. Good vertical coherence of the oscillations was

found between 150 and 600 m. Associated with these

oscillations are quasiregularly spaced thermohaline

fronts, which occur at half wavelength intervals (about

ZOO km). These fronts are strongest between 150 and

600 m and then weaken rapidly with increasing depth.

The origin of these quasiregularly spaced oscillations

is obscure; both Rossby wave generated by the annual

oscillation of the wind stress and instabilities in

baroclinic shear flow are capable of generating the observed wavelengths (Kang and Magaard 1982).

The amplitude of the mesoscale thermohaline perturbations in the western basin does not appear to depend significantly upon season. In Figure 29 is shown

the thermal structure along 167° E in the summer of

1982 (Roden and Taft 1984, 1985) _The subarctic front

is located near 42°50' N. A remnant of the winter

mixed isothermal area to the south of this front is visible between about 200 and 300 m. In the main thermocline there are quasiregularly spaced oscillations

with amplitudes of about 200-300 m and dominant

wavelengths of 300 to 500 km.
Roden: Subarctic-Subtropical Transition Zone of the North Pacific 21

4

T(OC) 16-23 JUL 1986

4

·1· _4

16-22 JUL 1985

400

500

600 '\ (/)

a::

w 700

w T (OC)

::2: 800

z 0

:c

100 w 3' :r: 0

200

300

400

4

500 "\3 600 ........ "-

3

700 ..\. . . . . .. .

T(OC) 16-22 JUL 1984

800

300

LATITUDE AT LONGITUDE 1700 E, R/V HOKUSEI MARU

480 460 44°N 42° 40° - 48° 46° 44°N 42° 40° 48° 46° 44°N 42° 40° 38°

O~~.~~~

Figure 21

Interannual variations of the subarctic temperature front in the western Pacific basin. Based on Hokkaido

University (1982-1987) data reports.

Eddy and Wavelike Perturbations in the Eastern

Basin-The eastern North Pacific between 30° Nand

50° N is dominated by a low-amplitude eddy field

superposed upon a weak mean flow (Wyrtki et al.

1976). The thermohaline perturbations in this region

are lower by factors between 3 and 6 compared with

those observed at comparable latitudes in the western

Pacific. Typical temperature, salinity, and density sections across the transition zone at 158° Ware shown in

Figures 30-32. The sections are based on a 27-km station spacing and were occupied in early fall 1975. Several noteworthy features stand out. The transition zone

is most clearly defined in the salinity field and is located

between about 34° Nand 39° N. On its northern side,

between 39° and 43° N, lies the subarctic frontal zone

in which several salinity fronts are located. On its southern side, between 32° and 34° N, lies the subtropical

frontal zone, in which two haline fronts are encountered. The salinity fronts are most pronounced in the

upper 200 m and have strong surface signatures. Beneath 200 m, the isohalines undergo low amplitude

(up to 50 m) oscillations of variable wavelength and

show few frontal features except in the vicinity of the

Hawaiian Ridge at the southern end of the section.

The frontal zones and fronts are much less defined

in the corresponding temperature and density sections.

Summer heating has led to the formation of an intense

shallow thermocline between 30 and 60 m that
22 NOAA Technical Report #105

LATITUDE AT LONGITUDE 170oE, R/V HOKUSEI MARU

44°N 42° 40° 48° 46° 44°N 42° 40° 48° 46° 44°N 42° 40° 38°

500 ~t\::'.:: / 34.0

••.3~.2\ .

.. ~.

S (%o) 16-24 JUL 1983

34.2

..~. S (%o) 16-22 JUL 1982

500

~. 34.0

600 . .. .

34.2

700 . \ .. J.

S (%o) 16-22 JUL 1984

800

S i¥ ~:: ... ·34~ . 800LS_(crc_o_o}_1_6_-_2_2_J_U_L_19_8_1-,

z 0 f--+-r-.,L.....J......,-l""rt-~-+-.....L-,--"---i

I

Ia..

W

o

Figure 22

Interannual variations of the subarctic salinity front in the western Pacific basin. Based on Hokkaido

University (1982-1987) data reports.

stretches as an unbroken band from 47° N to 27° Nand

only begins to weaken in the tradewind region to the

south. In the heated surface layer, horizontal tempera-

~ure gradients are weak, rarely exceeding 1-2° C/

30 km. These weak gradients nearly balance the existing horizontal salinity gradients such that density fronts

rarely form. Beneath the intense seasonal thermocline

and a depth of 200 m, the subarctic frontal zone is

outlined by weak thermal fronts in which the 8° C and

10° C isotherms are embedded. The subtropical frontal

zone is characterized by the rapid deepening of the

14-18° C isotherms, from about 60 m to about 200 m at

32° N. Below 200 m, the isotherms and isopycnals undergo low amplitude oscillations. The amplitudes are

fairly uniform throughout the transition zone, then increase southward to the Hawaiian Ridge.

The positions of the transition and frontal zones do

not vary much, when judged from the salinity distributions.· In the spring of 1968, the transition zone was at

the same location as in the fall of 1975. The subarctic

frontal zone in the spring of 1968 (Fig. 8) was slightly

wider than in the fall of 1975, extending from 39° N to

45° N. This was also the case for the SUbtropical frontal

zone, which extended from 31°-34° N in the spring of

1968 as compared to 32°-34° N in the fall of 1975. Investigations of the subtropical frontal zone in the winter of 1974 also showed it to lie between 31° at 158° W

(Roden 1980).
Roden: Subarctic-Subtropical Transition Zone of the North Pacific 23

\ .... ,

..

T(OC) 25 JUL-1 AUG 1983

25-30 JUL 1985 T (OCl 24- 30 JUL 1986

:~ 23-31 JUL 1981

4

700 .~

T (OCl 24-30 JUL 1984

500

600

800

300

400

enffi 700

t;j T (OC) :E 800L-----------..J

: °n~~~~~i~~~~ b:: 100

w

o

200

LATITUDE AT LONGITUDE 175°30'E, RIV HOKUSEI MARU

48° 46° 44°N 42° 40° 48° 46° 44°N 42° 40° 48° 46° 44°N 42° 40° 38°

O~~~~~

Figure 23

Interannual variations of the subarctic temperature front in the eastern Pacific basin. Based on Hokkaido

University (1982-1987) data reports.

Inside the frontal zones, convoluted fronts are found.

A three-dimensional study of the subtropical front in the

winter of 1980 (Roden 1981; Van Woert 1982; Niiler and

Reynolds 1984) revealed a persistent cold meander centered near 300 N 1530 W. The meander was identifiable

by satellite infrared images, shipboard derived sea surface temperatures, surface dynamic topography relative

to 1500 decibars (db) and satellite tracked drifter trajectories (Fig. 33). A coldcore cyclonic eddy was observed

to the north and a warmcore anticyclonic eddy was observed to the south of the front. Drifters released north

of the front tended to stay north and those released

south tended to stay south. The front changed slowly

with time, as indicated by salinity (Fig. 34). While the

frontal position and shape remained relatively constant,

its intensity increased by the advection of low salinity

water from the north.

Topographically induced mesoscale variability-The

ocean floor in the region of the subarctic-subtropical

transition zone is not flat, but contains topographic features of various shapes and sizes. Some of these are

minor abyssal hills and valleys, but others rise abruptly

from the deep ocean floor to a height of 4 to 5 km.

These large amplitude seamounts can occur singly, in

clusters, or arranged along planetary scale seamount

chains.

The tall seamounts reach into the stratified part of

the ocean and interact with the impinging flow which,
24 NOAA Technical Report #105

,---------------------------------------,

LATITUDE AT LONGITUDE 175°30'E, RIV HOKUSEI MARU

44°N 42° 40° 48° 46° 44°N 42° 40° 48° 46° 44°N 42° 40° 38°

200

~·3~.8 .

300 -' .~•..

.34.0.

..8.

...

. . . . 34.0

..

~·v······ 34.0

. .. . . . . . ..

34.2 .

S (%0) 24-30 JUL 1986

7\ . S (%0) 25 JUL-1 AUG 1983

........

~.34.2

.... . S (%0) 25-30 JUL 1985

~.-:"3~.?~.

S (%0) 24-30 JUL 1982

.....

34.2

.... ~. ./'

S (%0) 24-30 JUL 1984

...3.4~.

......~.

700

500

800

600

600

400

300

:I: h: 100

w

o 200

.

(/) 34.2

[5700 ... .

t;J S (%0) 23 -31 JUL 1981

800 '------------>---'

Z 0 r--'--~\'n._-'---+--..........+.-+---,..J....,.__1

Figure 24

Interannual variations of the subarctic salinity frunt in the eastern Pacific basin. Based on Hokkaido

University (1982-1987) data reports.

in general, is both time variable and spatially

nonuniform. The results of these interactions are

highly complex. They affect the ocean circulation on

all scales and are known to enhance boundary mixing

(Armi and Millard 1976; Eriksen 1985), amplify tidal

flow (Meincke 1971), generate internal wavetrains

(Eriksen 1982), give rise to eddies and Taylor columns

(Owens and Hogg 1980; Roden 1987), trap drifting

buoys and advected eddies (Cheney et al. 1980), cause

mesoscale perturbations of sea surface height (Roden

and Taft 1985) and deflect boundary curren ts such as

the Kuroshio (Roden et al. 1982; Vastano et al. 1985).

These physical processes have biochemical and biological consequences, ranging from nutrient enrichment

and enhanced primary productivity to concentrations

of demersal fish on and around seamounts (Boehlert

and Genin 1987).

Many of the above processes are amplified in the

vicinity of the Emperor Seamount Chain, which connects the Aleutian and Hawaiian island arcs in the

neighborhood of 1700 E. In Figure 35 are shown the

deflections of the Kuroshio and subarctic currents and

thermohaline fronts by the seamount chain, at a depth

of 150 m. This depth was selected to bring out more

clearly the various fronts and eddies. The Kuroshio extension, located near 350 30' N at 1670 E, approaches

the seamounts in a southeastward direction. About 50

km to the west of Kinmei Seamount (330 43'
Figure 25

Mesoscale thermohaline structure in a zonal section along 40° N off Cape Ma Saki, Japan. After

Roden (1975).
45

90

135

180

225

2701

315j

I

360

405

450

495

15 540 ....

w

'" 585

J: 630

t 675

720

765

-810

855

900

945

990

1035

1080

1125

1170

1215

(

I

/\ /3-.

,6~o

4~

,~

a>

z

/')

go

[

"0

o.,

...

=1*=

....

o

""

Figure 26

Meridional thermal structure across the transition zone at 1680 E in April 1971.
45

135

lBO

225

270

315

360

405

450

495

540

5B5

630

675

720

765

1035

lOBO

1125

1170

121

2l"N

!V\/4~(/~~ Figure 27

Meridional salinity structure across the transition zone at 168

0

E in April 1971.

1i' ?

rJJ

g.

C".

g. a

"0[

:;3

e: o=No=o...,

gozo

...

go

"C

o.

:l

r'l

-..J
~'

27.2~

27A~

n~

00

z

0

39° 38° -"7° 36° 35°N 34° 33° -,,?O ~lO -..rPN ?q' ?R" /70 ?f;0 ?'i0 ?4° ?-"O ??O ?_, I n

[

n

e.

'tl

0

::1

=II:

11'

0-

r\

Figure 28

Meridional density structure across the transition zone at 1680

E in April 1971.
Roden: Subarctic-Subtropical Transition Zone of the North Pacific 29

0

100

200

300

400

500

If)

a:

'" 600 a::l

U

w

0 700

;:::

w BOD

'"::::>

If)

If)

w 900

'"

(L

lOOO

liDO

1200

1300 L-..-J

100 KM

1400

1500 TEMPERATURE (oG)

JUN/JUL 1982

/

Figure 29

Meridional thermal structure across the

transition zone at 1670 E in June/July 1982.

171°30' E), the Kuroshio extension is deflected to the

northward. Over this seamount the current performs

an omega-shaped loop and weakens in the process.

Downstream of the seamount, the Kuroshio extension

intensifies again, though not to its upstream strength.

The temperature and salinity fronts associated with this

current undergo the same large amplitude anticyclonic

deflections over the seamount. Similar deflections have

been observed by satellite tracked drifters (Vastano et

al. 1985).

The subarctic current also gets deflected by the Emperor Seamounts. This current, located near 41 ° N at

167° E, first flows toward the southeast, then turns

northward over the southern part of Nintoku Seamount (4P N, 170°40' E) and continues in that direction until about 42°30' N, whence it turns eastward and

splits. The southern branch of this current forms a

large omega-shaped loop returning to the southeast,

while the northern branch continues eastward. A

strong thermohaline front is located over this seamount in the vicinity of 43° N. The position of this

front is remarkably constant from year to year (Fig. 22),

indicating that it may be topographically trapped.

The flow topography interaction along the Emperor

Seamount Chain results not only in the deflection of

large scale flows, but also in the generation of large

amplitude mesoscale eddies, with roots that extend

deep. This is shown in Figure 36, which illustrates a

pronounced eddy 40 km west ofJingu Seamount. The

eddy is strongly asymmetric, has stronger gradients on

the seamount side', which are indicative of enhanced

baroclinic flow there. The core of the eddy is about 60

km wide. The largest vertical displacements of the

isopycnals are about 300 m and occur between 200 and

500 m. The amplitudes of these displacements decrease

rapidly on approaching the underside of the intense

seasonal pycnocline and somewhat more slowly on approaching the level of the seamount top.

Drifter trapping by the Jenkins Seamount group, located just south of the Emperor Seamounts, is shown in

Figure 37, based on data by Cheney et al. (1980). The

drifter, originally released in the Kuroshio near Japan,

arrived in the vicinity of this group on 30 March 1977.

It then performed numerous anticyclonic loops among

the seamounts of this group and departed the regioiJ.

on 25 May 1977. The loop diameters ranged from 5 to

65 km. Peripheral speeds depended on loop size,

reaching 0.7 ms-I in the large loops. The drifter trajectories are suggestive of transient eddies generated by

the seamount group.
450

1495

ffi 540

585

630

..

l!J 675

720

765

3 .

3

\/'\

(jO

Q

z

;;3,.,

[

!.

l

o

'1

...

'*l:

...

Q

Figure 30

Meridional thermal structure across the transition zone at 1580 W in early fall 1975.
SALINITY ('%oJ AT 158"W SEPT/OCT 1975

Figure 31

Meridional salinity structure across the transition zone at 1580 W in early fall 1975,

90

135

160

225

270

315

36

405

450

495

540

565

630

675

720

765

610

655

90;

945

990

1035

1060

112

11;){J

1215 ~~ o ~'~'

25° 22"N

Ii"

?

g'

...

Sa"Cr;"

e:.

&:

o

=

=

...,

o

z

o

;.

Si

n

-
\>:l

Nl

Z

I /'l 22"N [

/'l

e:.

:::tl

"1:1

0

::1.

:Il:

....

0

\l<

I ~

v

~~26.5r-"

~~~~'-./'~~,~~~A9 , 'Jf\r-/';J ~~.0~~'-j-J 270 '

72

7.4~ ~27.4~

27.0.

765

248 45

90

135

180

225

270

315

360

405

900

810

855

945

1215

Figure 32

Meridional density structure across the transition zone at 1580 W in early fall 1975.
Roden: Subarctic-Subtropical Transition Zone of the North Pacific 33

after Niiler

32° ,..---r---,-----,---,----,..---.

N 31 JAN -11 FEB 1980

R.V.

32° .---.---,------,r------.,----,---,

N

29° 29° 155°W 152° 151°

100 km

32° 32°

N 31 JAN -11 FEB 1980 N 23 JAN -15 FEB 1980

R.V. WASHINGTON ro (C) DRIFTING

BUOYS

31° 31°

Figure 33

The subtropical front north of Hawaii during the winter of 1980 as seen by satellite infrared image (top left),

ship-derived sea surface temperature (bottom left), surface dynamic typography relative to 1500 db (top right),

and satellite tracked drifting buoy (bottom right). After Roden (1981), Van Voert (1982), and Niiler and

Reynolds (1984).

Conclusions and Discussion _

In the classical view, the subarctic-subtropical transition zone is a region between the subarctic and subtropical gyres in which the flow is orderly to the eastward and across which the thermohaline properties

change smoothly (Sverdrup et al. 1942). The view is

reinforced by the large scale distribution of plankton

species, many of which show a distinct zonation of subarctic, transition zone, and subtropical types (Brinton

1962).

The classical view changed when high resolution

physical and biological sampling techniques replaced

the low resolution hydrographic casts at widely spaced

stations. In the modern view, the subarctic-subtropical

transition zone appears as a complex region in which

mesoscale flow structures and biological patchiness

dominate at any instant of time. This is evident in both

shipboard and remote sensing investigations. Fronts,

jets, eddies, current meanders, and wavelike perturbations dominate the physical field (Kawai 1972; Roden

1975, 1977a, '1981; Cheney et al. 1980; Vastano and

Bernstein 1984). Plankton populations respond to

these temporally varying flow structures, partly following and partly migrating across them (Lobel and

Robinson 1986; Woods 1988). Seamounts and seamount chains add to the mesoscale variability by entrapping, generating, and shedding eddies and maintaining resident demersal populations (Boehlert and

Genin 1987; Roden 1987).

The observed mesoscale structures are the result of

interactions on all scales. Large scale flows such as the

Kuroshio extension and the subarctic current grow unstable, and shed eddies. Mesoscale eddies are generated also directly by traveling and stationary atmospheric disturbances and the irregular b~ttom

topography. Once formed, the mesoscale eddies interact among themselves and with the mean flow. Transientjets are developed between the eddies and on the
34 NOAA Technical Report #105

1520

S%o 31 JAN-11 FEB 1980

152° 1550

LONGITUDE WEST

1540

S %0 24-30 JAN 1980

1550

29°

31° 31 °

I

I--

a:::

0

z

w

0

::J 30° I--

I--

e:{

...J

Figure 34

Temporal changes of the subtropical salinity front north of Hawaii during the winter of 1980. Dotted

pattern indicates cold, low salinity water, ~triped pattern warm, high salinity water. After Roden (1981).

36"

N

36"

34"

16' :

14~ 32" TOG

150m

38"

N

36'

34"

32'

176<>

:~ ....

3Z"34~ . 34.4 /~

/ 34',45 ~"<)

... ~\'

... ,,~~ • . .: 34.7 .:

4~ 32" 5%0

150m 1982

36'

N

36'

34"

32'

36'

N

36"

168'

38'

N

36'

34"

32'

Figure 35

Deflection of the Kuroshio and subarctic thermohaline fronts and baroclinic currents by the Emperor Seamounts. Dots refer to

stations taken by the RV Thomas G. Thomas and triangles by the RV Oshoro Maru, both in the summer of 1982. After Roden

(1987), based on data by Roden and Taft (1984) and Hokkaido Cniversity (1983).
Roden: Subarctic-Subtropical Transition Zone of the North Pacific 35

2800 TEMPERATURE I'G)

2000

~800 SIGMA-T (kg/m3)

2400

2200

2600

'-'

z

...,

L-...J

100 KM

2000

2200 346~ \...rSALI~ (%0) n N "\ ~800 '---/ ........, ""

2600

2400

AT LATITUDE 38°40'N. JUNE 1982

167" 168" 169' 170' 171 172'E 167" 168' 169' 170' 171' 172'E I, [ I., 1,1 I !!!.I","!" I!, I" 1" t .".["".I,!

o LJ ~34A 0 24.~24.9

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::: :::~342 :::265.

:::342~ ::: :::11

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1400 "'-...- 1400 "----

345~ \ 275 n.(\ \ 1600 "-- 1600 - . '---

1800 1800 276~ V

2400

2000

2600

1800

1400

1300

1600

<J)

Q:

gj 900

2200

700

600

800

500

400

300

200

100

o

u

UJ

Cl 1000

:5

UJ 1100 Q:

::J

<J)

1200 Q:

CL

Figure 36

Mesoscale disturbances induced by Jingu Seamount. Note the sharp uplift of the isotherms, isohalines, and

isopycnals beneath the pycnocline.

flanks of these jets are concentrated submesoscale vortices, which are accompanied by upwelling and

downwelling patches a few kilometers across. The upwelling patches may include nutrient enhancement

with increase in primary productivity, i.e., represent biological patchiness (Woods 1988).

Analysis of recent and historical data permits one to

draw the following conclusions:

• The subarctic-subtropical transition zone is an

oceanwide feature associated with the oceanic and

atmospheric gyre scale circulations and air-sea energy

exchange processes.

• The transition zone is bounded in the north by the

subarctic frontal zone and in the south by the subtropical frontal zone. In each, one or several meandering fronts and frontal eddies are found. In the

upper layer of the ocean, temperature, salinity, and

density fronts do not always coexist.

• In mid-ocean, the positions of the transition and associated frontal zones are relatively fixed in time and

space, varying not more than 150-200 km from their

climatological mean positions. Individual fronts

within these zones are highly time variable, however.

• The northern boundary of the transition zone is

marked by the disappearances of the subarctic halo-
36 NOAA Technical Report #105

174°

31°

JENKINS

SEAMOUNT

%~

(Cheney et 01,19801

29°L----L- ..l.- 129°

171 0 172°E 173 0 174 0

Figure 37

Path ofsatellite tracked drifters in the vicinity of the

Jenkins Seamoun t group. After Cheney et al. (1980).

cline and the overlying shallow temperature minimum. Large changes occur in the hydrostatic stability structure. A seasonal stability gap develops near

the northern boundary of the transition zone, allowing atmospheric influences to penetrate deeper than

elsewhere in the region during winter.

• The southern boundary of the transition zone is

marked by the disappearance of the subtropical halocline and by large changes in the vertical structure of

the thermocline and pycnocline, which deepen toward the south.

• Mesoscale perturbations of the flow and thermohaline structure are the dominant features on synoptic

time scales, greatly overshadowing the weak mean

flow and the climatological mean structure. The mesoscale perturbations west of the Emperor Seamount

Chain are three to six times larger than those to the

east.

• Mesoscale perturbations in the western basin are

characterized by alternating bands of eastward and

westward flows 150-300 km wide and vertically coherent in the upper 1500 m. The corresponding variations in sea surface height are of the order of 0.4 m

throughout the transition zone. The mesoscale flow

perturbations are accompanied by quasiregularly

spaced temperature and salinity fronts in the thermocline and halocline.

• Mesoscale perturbations in the eastern basin are characterized by alternating bands of weak eastward and

westward flow that does not penetrate much below

600 m. The corresponding variations in sea surface

height are of the order of 0.1 m in the transition zon('.

Because the mesoscale flow perturbations are weak,

quasiregularly spaced temperature and sal~nity fronts

in the thermocline and halocline are not eVIdent.

• Large amplitude topography has a measurable effect

on flow and thermohaline structure in the transition

zone. The interaction of the Kuroshio and subarctic

currents with the Emperor Seamount Chain not only

results in deflection and weakening of these currents

as they cross the chain, but also results in the generation of intense mesoscale eddies. Thermohaline

fronts accompany these current deflections and eddies and extend to depths of 1 km or more. The

fronts are strongest beneath the seasonal thermocline and are not always detectable at the sea surface

(in case of temperature).

• The interannual variability of fronts depends on the

flow dynamics at the particular geographic location.

The subarctic front over Nintoku Seamount (430 N,

1700 E) appears to be topographically trapped. Over

the six-year period 1981-1986, the position of the

front varied by less than 50 km from its mean position at 430 • About 400 km to the east, over a flat

seafloor, the position of the same front varied by

more ~han 150 km during these years.

Acknowledgments _

The high resolution hydrographic data used in this

study were mostly collected by the RV Thomas G. Thompson during the years 1968-1982 under Office of Naval

Research contract N00014-84C-Oll1 (Roden and

Fredericks 1987, a through c) and National Science

Foundation grant OCE 81-17047 (Roden and Taft

1984, 1985). Data collected in the Emperor Seamounts

region by the RV Hokusei Maru (University of

Hokkaido, 1982-1987) made it possible to investigate

the interannual variability of the subarctic front. The

writing of this paper was supported by Joint Institute of

Marine Research (JlMAR) of the University of Hawaii.

Special thanks are due J. Wetherall of the Honolulu

Laboratory of the National Marine Fisheries Service for

organizing the NMFS/JlMAR Workshop on the Biology, Oceanography, and Fisheries of the North Pacific

Transition Zone and Subarctic Frontal Zone in May

1988 and for inviting the author. This is contribution

No. 1871 from the School of Oceanography, University

of Washington, Seattle WA 98195.

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Springer-Verlag, NY.
Roden: Subarctic-Subtropical Transition Zone of the North Pacific 37

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1987. Hydrographic sections across the Kuroshio extension

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1978. A survey of world-wide sea surface temperature fronts

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1983. Annual and interannual variability in the Kuroshio

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1981. Direct atmospheric forcing of geostrophic eddies.

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38 NOAA Technical Report #105

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81: 2641-2646.
Biology of the Transition Region

WILLIAM G. PEARCY

College of Oceanography

Oregon State University

Corvallis, OR 97331

ABSTRACT

The transition region of the North Pacific Ocean includes the Transitional Domain of

the Subarctic Water Mass and the transition zone south of the subarctic boundary. It is

therefore located between two different epipelagic ecosystems, each with characteristic

nutrient regimes, productivity cycles, and faunal compositions. The transition region has

distinct endemic species of zooplankton and micronekton. Common species of epipelagic

nekton caught in gill nets both north and south of the subarctic boundary in this region

during the summer include Pacific saury, Pacific pomfret, albacore, blue shark, flying

squid, and boreal clubhook squid. Albacore, skipjack tuna, and dolphin are common

south of the subarctic boundary in the transition zone. Most of the large nektonic animals

of this region undertake north-south seasonal migrations that capitalize on the high

productivity of the subarctic waters in the spring and summer and the availability of large,

nutritious prey such as gonatid squids. During the winter, migrants retreat south of the

subarctic boundary where water temperatures are warmer, the seasonal changes in productivity are less pronounced and frontal structure may concentrate prey. In addition to

preying on the endemic fauna of the subarctic region in the summer, the large migratory

fishes and squids also consume small migratory fishes, such as saury and sardine, that

precede them in northward movements.

Transition Region

Two predominant gyres exist In the orth Pacific

Ocean: The cyclonic subarctic gyre north of 45°N

and the anticyclonic North Pacific central gyre in

subtropical latitudes. Distinct water masses and biota

are associated with these gyral systems. The subarctic

boundary, often denoted by the vertical 34%0

isohaline (Dodimead et al. 1963; Favorite et al.

1976), usually forms a sharp front along the southern

boundary of subarctic waters between 40-45°N across

the Pacific Ocean (Fig. I). Cool, low salinity waters

with a permanent halocline between 100-

200 m and large seasonal variations in temperature

in the upper mixed layer characterize subarctic waters north of this front. South of the subarctic

boundary, between 42° and 32°N, lies the transition

zone, a zone of mixing between subarctic and central

waters where the halocline is weak and stability is low

(Roden 1970, 1980). The subtropical front is a region

of rapid, southward increases in temperature and salinity in the winter and in salinity in the summer

along the sou'thern margin of the transition zone,

between the prevailing westerlies and easterly trades

at 28°-35°N (Roden 1975). North Pacific Central

Water south of this front is characterized by warm,

high salinity (35%0 at the surface) water with a persistent thermocline, high stability, no halocline

below low-salinity surface waters, but a salinity minimum at about 500 m (Roden 1970, 1980).

The Subarctic Water Mass has been divided into

several domains (Dodimead et al. 1963; Favorite et

al. 1976). The transi tional domain is located between

the subarctic boundary and the central subarctic domain (Fig. I). Here the flow is principally zonal. The

northern boundary of the transitional domain has

been defined by several physical features: where the

7° isotherm intersects the upper halocline

(Dodimead et al. 1963), the surface isohaline is

33%0 or the 4°C isotherm plunges below 100 m (Favorite et al. 1976; Shimazaki 1986), or where

dichothermal or temperature minimum layer of the

more northern central subarctic waters disappears

(Dodimead et al. 1963). The transitional domain is

latitudinally narrow in the western Pacific and broad

in the eastern Pacific where it expands into the Cali39
40 NOAA Technical Report #105

40'

35'

65'

55'

50'

45'

30'

180' 170' 160' 150' 140' 130' 120'

...:

"

...'

150' 160' 170'

CENTRAL SUBARCTIC DOM~' !j\~E~~i

:'\N£.':>\~~~~ .;}.

TRANSITION REGION TRANSITIOt ZONE -I

J _--- SUBTfWPICAL FRONT --------------i_I'~v

---------- ----------- NORTH PACIFIC CENTRAL WATER

140'

40'

35'

30' •

25' ~=====================================:=H25'

140' 150' 160' 170' 180' 170' 160', 150' 140' 130' 120'

Figure 1

Oceanographic regions of the lorth Pacific Ocean.

fornia Current and Alaska Current. The ridge domain, with doming of isopycnals indicative of massive

divergence, is found to the north of the transitional

domain in central subarctic waters.

Because many of the nektonic animals associated

with this region between waters of the central subarctic and central North Pacific are broadly distributed

and migrate between the boundaries of these two

gyres, this review includes both the transition zone

and the transitional domain. 1 will call both regions

combined the "transition region" (see Fig. I).

Biology of the Transition Region

and Adjacent Waters _

Since the transition region is an area of mixing between Subarctic and Subtropical Central Water

Masses, our understanding of this region is faciEtated

by comparison of the basic features of these two different gyral ecosystems that border the tramition

region (Table I).

Primary Production and Plankton Dynamics

Concentrations of nutrients, such as P04-P, are generally high in subarctic waters and low in the

subtropical waters (Reid 1962; McGowan and Williams 1973). Strong north-south gradients in nitrate,

phosphate, and silicate are found in the region of the

subarctic front between these water masses (Park

1967; McGowan and Williams 1973). Phytoplankton

standing stocks, as measured by chlorophyll a, and

productivity also decrease markedly from eutrophic

subarctic waters across the transition region into

the subtropical regions (Koblenz-Mishke 1965),

McGowan and Williams (1973) found that mean concentrations of chlorophyll a were significantly lower

in the transitional domain than in central subarctic

waters during both summer and winter seasons.

Phytoplankton production in oligotrophic central

waters is limited by lack of major nutrients resulting

from the weak mixing and strong vertical stability of

the water column throughout the year (Eppley et al.

1973; Hayward et al. 1983). There is little evidence
Pearcy: Biology of the Transition Region 41

Table 1

Features of the structure and function of plankton in three geographic regions of the North Pacific Ocean.

Phytoplankton

Limiting Factors

Chlorophyll a

Seasonality in chlorophyll

Primary productivity (g/m2

/yr)

Productivity peak

Summer/winter ratios in prod.

Zooplankton

Seasonal variability

Standing stock

Summer/winter (g wet/l000m3)

Carnivore: total zooplankton

Body size of macrozooplankton

Lipid content

Seasonal vertical migrations

Species richness

Percent endemic species

Central

subarctic

Light, Fe

Grazers

High

No

100-150

Spring

>10

High

High

200-500/<50

Low

Large

High

Yes

Low

High

Transition

region

N

Mod.

No

80-100

Late Spring?

2-6

Low

Mod.

100-200/50-100

?

Mod.

Mod.

North Pacific

central

Nand P

Low

No

<50

Winter?

2

one

Low

10-50/10-50

High

Small

Low

No

High

Low

References

1,2,3,4

5,6,7

8,9

1,9,10

II

5,9

II

5,9,10,12

9, II

II

11, 13

8,14

14

10, ll, 15

13, 14, 16, 17

16

1 Parsons and LeBrasseur 1968

2 Anderson et al. 1969

3 Eppley et al. 1973

4 Martin and Fitzwater 1988

5 Frost 1983

6 Miller and SUPER group 1988

7 Miller et al. in press

8 Reid 1962

9 McGowan and Williams 1973

for seasonal cycles of chlorophyll concentrations of

primary productivity (McGowan and Williams 1973;

Taniguchi 1981; Hayward et al. 1983). A deep chlorophyll maximum is present above the nutricline in the

region of the upper thermocline during all seasons

(McGowan and Williams 1973; Venrick et at. 1973).

In the subarctic Pacific, on the other hand, where

deep water is upwelled and intense winter mixing

above the halocline alternates with thermal stratification in the euphotic zone, major nutrients are not

depleted, even during the spring and summer

(Anderson et at. 1969). Large seasonal fluctuations

exist in primary productivity. Production is high in

the spring when nutrients exist in high concentrations above the shallow thermocline. But this region,

including the transitional domain, is characterized by

a lack of phytoplankton blooms (Parsons and

LeBrasseur 1968; McAllister 1969; Parslow 1981;

Taniguchi 1981; Frost 1983).

10 Hayward et al. 1983

II Taniguchi 1981

12 McAllister 1960

13 Reid et al. 1978

14 Motoda et al. 1974

15 Miller et al. 1984

16 McGowan 1974

17 McGowan and Walker 1985

Phytoplankton standing stocks in the subarctic Pacific, which are dominated by nanoplankton

(Parsons and Lalli 1988), are limited by low temperature and light intensities, by grazing of herbivores in

the spring, (McAllister 1960; Venrick 1971;

Taniguchi 1981; Frost et at. 1983; Miller and SUPER

group 1988; Parsons and Lalli 1988), and by low

availability of iron (Martin and Fitzwater 1988; Martin et at. 1989). The absence of a spring bloom in the

subarctic Pacific had been attributed to intense grazing by the large copepods, particularly the endemic

Neocalanus spp. which reproduce without feeding in

the spring, using lipid stores accumulated from the

previous growing season (Heinrich 1962; Frost. et at.

1983; Miller et at. 1984). Recently, the role of

microzooplankton in this ecosystem has been emphasized, and now heterotrophic protozoans, not

large calanoid copepods, are thought to be major

consumers of nanoplankton in subarctic waters
42 NOAA Technical Report #105

(LeBrasseur and Kennedy 1972; Frost 19H7; Miller

and SUPER group 1988). The absence of a typical

phytoplankton bloom in the subarctic Pacific may

therefore be due to iron limitation, which establishes

a phytoplankton community dominated by small

cells, intense grazing by microzooplankton during

the spring, and the reduced mixing of the water column by the strong halocline which retains both

phytoplankton and grazers in upper waters during

the winter (Miller et aI., in press). The southern extent of the "balance" phenomenon is not known, but

nanoplankton and microzooplankton are presumably

the predominant producers and grazers throughout

the transition region.

The geographic pattern of zooplankton biomass in

the North Pacific is very similar to that of phosphates

(Reid 1962) with high standing stocks in subarctic,

low standing stocks in central waters, and a pronounced gradient across the transition region

(Hayward et al. 1983). A region of lower than average

standing stocks of zooplankton noted by Reid (1962)

in the central subarctic domain is attributed to rates

of surface divergence that exceed the population

growth rates of zooplankton.

McGowan and Williams (1973) reported higher

standing stocks of zooplankton (0-300 m) in the central subarctic domain than in the transitional domain

during summer and winter. Seasonal cycles of zooplankton biomass are weak in central waters (Hayward

et al. 1983) but pronounced in subarctic waters.

Taniguchi (1981) estimated that the summer: win ter

ratio of zooplankton in the upper 150 m was 4-10 in

subarctic waters compared to 1.0 in central waters.

McAllister (1960) and Frost (1983) also reported large

seasonal variations in the standing stocks of zooplankton at Station "P" in the subarctic Pacific.

The ratio of carnivores to total zooplankton also

differs between these two gyral ecosystems, with carnivores composing a larger fraction of the total in

subtropical areas (Reid et al. 1978; Taniguchi 1981).

The average body size of zooplankters is larger in

subarctic than in subtropical waters, and their lipid

content is higher in northern areas (Motoda et al.

1974). Seasonal or ontogenetic migrations of zooplankton are pronounced in the .subarctic Pacific.

Neocalanus copepods enter a diapause state and descend into deep water in the summer (Miller et al.

1984; Miller and Clemons 1988). Such ontogenetic

migrations of common zooplankton are not well

documented from the transition or SUbtropical regions. Diel vertical migrations of zooplankton and

micronekton are probably more pronounced in central waters where transparency is high and day-·night

cycles of sunlight are more equal (Hayward et al.

1983).

Less is known about the plankton dynamics of the

transition region than for either of the adjoining

gyres (McGowan and Williams 1973). McGary et al.

(1958) concluded that the transition zone was a region of low vertical stability where optimum conditions for a phytoplankton bloom and increased

zooplankton abundance would advance to the north

in the spring.

Micronekton

Micronekton also' increases from central to subarctic

waters. Average volumes of small fishes, squids and

crustaceans in Isaacs-Kidd midwater trawls were

higher from 35° to SOON in the Subarctic Currenttransitional domain than between 28° and 35°N in

the transition zone north of Hawaii (King and

Iversen 1962). Pearcy et al. (1988) found that the

biomass per m 3 of taxa collected in a 4.5 m 2_

midwater trawl was larger in subarctic than in

subtropic waters with large catches of medusae and

ctenophores in the central subarctic domain.

Donaldson and Pearcy (1972) also reported large

variations In the biomass of plankton and

micronekton captured in a 3-m Isaacs-Kidd midwater

trawl in the mid-Pacific between Hawaii and Adak,

Alaska. They found that zooplankton biomass was

largest in subarctic waters, intermediate in the transition region and lowest in central waters. The biomass

of fishes, shrimps and squids, however, was largest in

the transition region at 42°35.5' N.

These trends in catches were not always reflected

hy measurements of sound scattering at 38.5 kHz.

Subarctic waters had almost no scattering but high

biomass of most groups including euphausiids. Transitional waters had extensive scattering and a large

biumass of all groups. Central waters had extensive

scattering but low biomass of plankton and

micronekton (Donaldson and Pearcy 1972). Pearcy

(Fig. 2) noted that the number, intensity and maximum depth of 24-kHz scattering increased from the

central subarctic domain to the subarctic boundary

along 155°W during July. Increased scattering south

of the subarctic boundary may be explained by the

increased occurrence of good acoustical scatterers,

such as mesopelagic fishes with gas-filled swim bladders (Johnson 1977; Pearcy 1977).

Summary

The subarctic ecosystem, in contrast to the subtropic

ecosystem, is a cold, eutrophic region with large bio-
Pearcy: Biology of the Transition Region 43

D

,

."

~­~~ .....•

;' '. ~" '::f' '.~.:'••

. ;.

i[ L ,'..- '\

t

."

c

•

•

;r;::::·······

Figure 2

24 kHz Echograms recorded at mid-day from 55°30' N to 41 °40' along 155°W during July 1985, 0-400 m, 400-800 m.

mass of zooplankton and micronekton. Seasonal

fluctuations in production of phytoplankton and

standing stocks of zooplankton are large compared

to the warm, oligotrophic, southern waters where

species richness is high and seasonal variations are

low (McGowan 1974; Reid et al. 1978). According to

Hayward et al. (1983) and McGowan and Walker

(1985), the plankton of the cen tral gyre has low heterogeneity in community structure in both time and

space compared to regions like the California Current that are influenced more by advection, possibly

because a stable environment may create conditions

favoring strong regulation of species structure.

The transition region is poorly studied in comparison to the central and subarctic waters proper.

Gradients exist in physical and biological properties

across this region from the "end member" gyres to

the north and south. Fishes, squids, and shrimps appear more abundant there than in central waters. We

have little knowledge of the processes regulating the

productivity of this region (McGowan 1974) but can

assume the large-scale and mesoscale circulation interact with the biology to produce a unique

environment (Olson 1986). This environment may

also experience large interannual changes. For example, large-scale shifts in the subarctic boundary in

the eastern Pacific associated with EI Niiios may affect the species and size composition of zooplankton

in this region (Fulton and LeBrasseur 1985).

Taniguchi (1981) concluded that the subarctic region, with its high primary and secondary production and its large and nutritious zooplankton, is a

good foraging ground for fishes and other nektonic

animals during the spring and summer, but not during the winter when temperatures are cool and prey

organisms less available. The transition zone south of
44 NOAA Technical Report #105

the subarctic boundary is more favorable in the winter than summer because of moderate temperawres

and peaks in production during winter and spring

season. These may be major reasons for the pronounced seasonal latitudinal migrations of transition

region fishes and squids that will be discussed later in

this review.

The Transition Biogeographic Province _

Distributional patterns and species-associations of

oceanic plants and animals often have boundaries

that conform to water masses and their distinct physical, chemical, and biological properties. In the North

Pacific four major biogeographic provinces are usually recognized: subarctic, transition, central, and

equatorial (Fager and McGowan 1963; McGowan

1971,1974; Reid et al. 1978).

The transition region is delimited by the northern

and southern ranges of planktonic organisms

(McGowan 1971, 1974), the copepod Eucalanus

caliJornicus, and the euphausiid Thysanofssa gregaria

(Reid et al. 1978). All of these distributions extend

hundreds of miles both north and south of 400N, and

usually include both the transitional domain of the

subarctic waters as well as the transition zone south

of the subarctic boundary. In the eastern Pacific, the

ranges of transition species often extend as far south

as Baja California in the California Current.

Maintenance of endemic planktonic or even

micronektonic populations within the transiticn region with its easterly meridional flow is enigmatic.

Several hypotheses have been proposed to explain

the persistence of this unique zoogeographic pattern

(Willis et al. 1988):

1 a reproductive center of species in the western

Pacific with eastward dispersal of individuals in

the Subarctic Current;

2 return circulation in a counter current south of

400 N (McGowan 1971); and

3 reproduction throughout the transition region

permitting self-sustaining populations despite net

transport out of the area.

If the first hypothesis is true, youngest individuals of

a species would be found in the western Pacific, and

minimum sizes would increase to the west. This was

not observed'for mesopelagic fishes of this region. In

most· cases, smallest individuals were found in the

east and the minimum size increased to the west

(Willis et al. 1988). The pronounced mesoscale eddies often associated with the transition zone (Roden

1991), lend support to the third hypothesis for this

zoogeographic pattern.

Recent reviews of the oceanic zoogeography of

micronekton (mesopelagic fishes, shrimps and

squids) provide useful information on the fauna of

the transition region and its relationship to water

masses and other faunal regions:

Mesopelagic Fishes

Willis et al. (1988) found two basic patterns for mesopelagic fishes captured with small midwater trawls

north of 40° N in the subarctic Pacific: 1) those

found in the subarctic proper, and 2) those found in

the transitional domain. Within each of these general

regions, some species were mainly distributed a) in a

pan-Pacific zone across the Pacific, b) in a western, or

c) an eastern subarea of the region. Other species,

such as the abundant lanternfish Stenobrachius

lfucopsarus, were found throughout the entire subarctic and transition region. Similarity in the species

composition among regions was higher for fishes

sampled in deep (>500 m) than in shallow «500 m)

strata, indicating that the shallow mesopelagic fish

faunas are more provincial than the deep faunas,

probably because of the semiclosed gyres in the upper ocean.

In another study, Willis (1984) identified three faunal groups of mesopelagic fishes in the Northeast

Pacific: a) subarctic (north of the subarctic front), b)

central (south of the subarctic front and north of the

north equatorial front where salinity increased rapidly), and c) southern (near the Hawaiian Islands

south of the north equatorial front). The subtropical

front was not detected as a boundary for vertically

migrating mesopelagic fishes, probably because it is a

feature of the upper 200 m.

Shrimps

Seven distinct zoogeographic distributions were recognized for pelagic shrimps in the North Pacific,

including (singly or in combination) subarctic, transition, central, and equatorial (Krygier and Wasmer

1988). The distributions of many species appeared to

be centered in gyres but overlapped water mass

boundaries. Thirteen species were classified as transitional. The common Sergestes similis occupied both

subarctic and transitional zone waters.

Cephalopods

Jefferts (1988) identified zoogeographical patterns of

pelagic cephalopods that often corresponded to basic

water masses in the northeastern Pacific Ocean.
Transitional endemics ranging both offshore and in

the California Current region were identified. Most

of the 14 offshore transitional species were in the

family Enoploteuthidae, whereas 90% of the -subarctic species were Gonatidae. Species diversity and

richness were highest in central waters, but were

higher in subarctic than transition waters because of

the large number of endemic species of gonatids in

the subarctic region (Okutani et al. 1988).

Epipelagic Nekton _

Because of extensive fishing for salmonids and squid

by research vessels with surface gill nets, data are

available on the distributions and species associations

of large epipelagic animals in the North Pacific, especially during the summer season.

Mishima (1981) reported on the catches in research gill nets from about lat. 39°N to 50 0

N,

between long. 165°E and 175°E during the summers

of 1978 and 1979. Based on the catches of nektonic

animals in each set, these species assemblages were

identified: 1) subarctic animals which were found almost exclusively north of the subarctic boundary,

including six salmonids (Oncorhynchus spp.), eightarmed squid (Gonatopsis borealis), boreal clubhook

squid (Onychoteuthis borealijaponica) and salmon shark

(Lamna ditropis); 2)subarctic-transitional zone species

that were caught in waters both north and south of

the subarctic boundary, e.g. Pacific saury (Cololobis

saira) , Pacific pomfret (Brama japonica) , neon flying

squid (Ommastrephes bartrami), blue shark (Prionace

glauca) and Pacific mackerel (Scomber japonicus); and

3) transition zone species usually caught south of the

subarctic boundary, such as albacore (Thunnus

alalunga) , skipjack tuna (Katsuwonus pelamis) and dolphin (Coryphaena hippurus). Mishima noted that the

subarctic-transitional group comprises species that

migrate north- ward across the subarctic boundary in

the summer and retreat to the south across the

boundary in the autumn when water temperatures

decline.

Shimazaki (1986) determined the similarity of species composition of nekton caught in gill nets in

subarctic and subtropic areas west of 180° long'. during the summer. He concluded that the communities

were related to oceanographic features. The

subtropic region included an association of skipjack

tuna, small flying squid, sardine (Sardinops melanostictus) (and in some years immature albacore and

large blue shark). In the subarctic region, north of

the subarctic boundary, larger Pacific saury, sardine,

large flying squid, large pomfret, and boreal

clubhook squid were included in two species associations. The central subarctic and transitional domains

Pearcy: Biology of the Transition Region 45

had many of the same species. Shimazaki observed

that small planktivorous fishes, such as saury and sardine, migrated across the subarctic boundary into

subarctic waters earliest in the year, followed by the

large sizes of carnivores, such as pomfret and flying

squid.

A demonstration of a similar latitudinal trend in

the catch composition of gill nets in the eastern Pacific (55°N to 36°30' N along 155°W) was provided

by the Japanese Training Vessel Oshoro Maru (Table

2). Again, three species assemblages are recognized.

The first group of subarctic species included six salmonids, salmon shark, spiny dogfish (Squalus

acanthias), and the eight-armed squid. All of these

species were confined to subarctic waters north of

the transitional domain as denoted by the intersection of the 7°C isotherm with the top of the

halocline (33%0 isohaline). The second group of

transitional-subarctic species included Pacific

pomfret, boreal clubhook squid, smalleye squaretail

(Tetragonurus cuvieri) , saury, blue shark, albacore,

and flying squid. All these species, except albacore

and flying squid, extended northward beyond the

transitional domain into the central subarctic domain, but only pomfret and boreal clubhook squid

penetrated far north into the ridge domain by midJuly. Most of these species were also caught south of

the subarctic boundary in subtropic waters. The

third group of species were skipjack tuna, bigeye

tuna (Thunnus obesus), billfishes, frigate mackerel

(Auxis thazard), dolphin, and yellowtail (Seriola

dorsalis). These species were found only south of the

subarctic boundary with the exception of skipjack

and yellowtail.

Catches in Gill Nets Fished for Squidl

These meridional comparisons, and reports (Larkins

1964; Robinson and Jamieson 1984; Murata and

Shingu 1985; Bernard 1986; Murata 1986, 1987;

Jamieson and Heritage 1987), suggest the Pacific

pomfret, Pacific saury, and blue shark are the most

common species caught in association with flying

squid in surface gill nets in transition waters during

the summer. Few data exist on the incidental catches

of fishes in drift gill nets set for squid in the winter

season, but Risher (1988) observed that saury, blue

shark, pomfret, and bonito (Sarda chiliensis) were

captured along with flying squid during November,

175° E and 160° E when all flying squid were caught

south of the subarctic boundary.

I Incidental catches ofseabirds and marine mammals are considered

elsewhere in this report.
46 NOAA Technical Report #105

Table 2

Catches of fishes and squids in gill nets along 1550 W by Oshoro Maru, 15-30 July 1984. (Only total catches of five or

more individuals of identified species are included unless indicated. a

45-30 44 42-30 41 39-29 38 36-30

2

2

36

1444 72 56 94 2

3 4

18 98 174 2

6626 1 625 349 4241 6

1 4 29 128

23 3 3 39 6

26 57 167 27 48 64

6 104 4

6 I

I I

3

6

3

3 63

Subarctic Ridge Dumain

Latitude a N: 55 54 53 52 51 !iO 49 48 47

Sockeye 263 357 171 169 8tl 52 33 50

Chum 294 316 209 183 42 1~! I 117 3tl " Pink 313 110 134 105 104 158 118 5

Coho 146 27 16 15 31 33 16 .~ 2

Chinook 3 I I 1 I I I

Steelhead 42 24 33 16 11 9 10 6 4

Salmon shark I I I

Spiny dogfish I I 2 I I

Eight-armed squid 4 10 2 7 9 5 6 6

Pacific pomfret 9 12 14 177 1040

Boreal c1ubhook squid tl 12 24 15

Smalleye square tail 5

Saury

Blue shark

Albacore

Flying squid

Skipjack tuna

Bigeye tuna

Striped marlin"

Shortbill spearfish"

Frigate mackerel

Dolphin"

Yellowtail

Transition Domain

Subarctic Current

SubTropic

Although maximal catches of skipjack tuna commonly occur south of the latitude of maximal flying

squid catches (Murata 1986), Dahlberg and Sigler

(1987) reported that catches of skipjack tuna exceeded catches of flying squid in gill nets at some

stations in the western Pacific. Skipjack tuna were also

common in catches west of 180° long. where they

were second by weight only to flying squid (National

Fisheries Research and Development Agency 1987).

Other epipelagic species are sometitpes caught in

large numbers along with flying squid. Murata (1986)

reported very high catches of sardine (Sardinops

,melanostictus) north of the area of peak catches of

flying squid, and high catches of Pacific mackerel and

skipjack tuna south of the region of high squid

catches along 175° E. Sometimes albacore is common

in surface gill nets fished for pomfret and squid. Albacore ranked second in weight to Pacific pomfret in

catches from April to June from 171° W to ] 31 ° W

(Fisheries Agency ofJapan 1983). In the eastern Pacific, large numbers of jack mackerel (Trachurus

symmetricus) were taken while gill netting for flying

squid Uamieson and Heritage 1987) or albacore

(Powell 1957).

The incidental catches of several fishes are substantial in the gillnet fishery for flying squid. LeBrasseur

et al. (1987) and Robinson and Jamieson (1984)

found that Pacific pomfret often were more numerous than flying squid in their gill net survey in the

eastern North Pacific (38°-45° N, 150°-180° W).

LeBrasseur et al. (1987) estimated that]00-300 X 106

pomfret may be discarded annually by the drift net

fishery for squid. Sloan (1984) also observed that

pomfret was a major bycatch in gill nets used for

flying squid off the coast of British Columbia, and

Light (1986) reported that pomfret were the most

abundant fish caught in gill nets and longlines from

a research vessel in the western Pacific (43°30' N50°30' N).

Temperature and Catches of Nekton

Several authors have examined the relationships between sea surface temperatures and catches of

epipelagic fishes and squids in the North Pacific.

Most species occur over a broad range of temperatures, but highest catch rates usually are within a

relatively narrow "optimal" range. Generally the ma-
Pearcy: Biology of the Transition Region 47

EFFORT

PACIFIC MACKEREL - YELLOWTAIL

PACIFIC

SAURY

6 7 8 9 18 11 12 18 14 15 16 17 18 19 28 21 22 23 24 25 26

Sea Surface Temperature

Figure 3

Relative occurrence of common

nekton in gillnet catches of

Japanese, Taiwanese, Korean, and

Canadian research vessels vs. sea

surface temperature (data compiled by J. Wetherall).

-

-

BOREALCLUBHOOK

SQUID

EIGHT-ARMED SQUID -

-

- --

-- --

-

ALBACORE

BLUE SHARK

FLYING SQUID

POMFRET

jor species found in the transItIOn region, such as

Pacific pomfret, Pacific saury, blue shark, jack mackerel, Pacific mackerel, albacore, and flying squid, are

found in warmer waters than salmon, steelhead,

salmon shark and eight-armed squid (Neave and

Hanavan 1960; Manzer et al. 1965; Naito et al. 1977;

Macy et al. 1978; Brodeur 1988). The temperature

that delimits these two groups is about 12° C, a temperature that often marks the boundary between

subarctic proper and transitional domain (Mishima

1981; Brodeur 1988).

The relationship between catch per effort in surface gill nets and ocean features was examined for

three co~mon epipelagic squids in the subarctic Pacific by Kubodera et al. (1983) and Kubodera (1986).

They found that flying squid, boreal c1ubhook squid,

and eight-armed squid sometimes had overlapping

temperature distributions, with flying squid at the

warmest and eight-armed squid at the coolest temperatures. Water temperatures where these species

were caught generally increased between May and

August, probably as a result of seasonal warming of

waters above the thermocline. These authors also

stressed the importance of vertical features of temperature and salinity in the distribution of squids.

They suggested that high catches of flying squid were

associated with the region of the subarctic boundary

and the thermohaline convergence zone between

transitional and western subarctic domains, perhaps

because northward migration of this squid is impeded along these barriers.

Latitudinal movements of flying squid may also affect the relationship between catches and sea surface

temperatures. Seasonal changes in the surface water

temperatures where jigging boats operated for flying

squid decreased from about 20° C in early summer to
48 NOAA Technical Report #105

less than 15° C in autumn (Murata et al. 1983), suggesting that squid migrated northward in to cooler

water as the summer progresses. Gong (1985) and

Gong et al. (1985) showed that surface temperatures

where peak gillnet catches occurred varied among

months and with latitude during the year, but peak

catches were most often at about 15° C.

Data on the occurrence of epipelagic nekton in gill

nets fished by Japanese, Korean, Taiwanese, and Canadian research vessels for flying squid again emphasize

the broad range of temperatures occupied by most

species (Fig. 3). Salmonids, boreal clubhook squid and

eight-armed squid occurred mainly in the cooler

range of temperatures, and yellowtail and skipjack

tuna occurred in catches at warmer temperatures. Flying squid, pomfret, saury were present in gillnet sets

over a temperature range of 15° C or more.

The relationships between sea surface temperatures and the catches of salmonids in the region of

the Japanese drift gillnet fishery for flying squid indiTable 3

Prey identified from stomachs of transition-region nekton caught in gill nets set by the TV Oshoro Maru along 155° W,

3-18July 1985.

Species

Flying squid

Pomfret

Albacore

Blue shark

Saury

Yellowtail

Smalleye squaretail

Salmon shark

Skipjack tuna

ON

Latitude

44

42-30

41

39-30

38

36-30

48

47

45-30

44

42-30

41

44

42-30

41

39-30

38

41

39-30

36-30

42-30

41

48

41

47

45-30

44

36--30

No

Examined

16

44

63

28

33

25

38

24

27

6

1

25

5

3

13

6

43

2

2

1

4

18

5

4

1

4

4

5

Stomach contents

almost exclusively Berryteuths anonychus, few fish scales

fishes (scales/otoliths), squid

fish remains, pteropods, heteropods, amphipods

pteropods, crustaceans, squids, fishes

fishes, isopod, squid

fishes, crustaceans

100% Bl'Tryteuthis anonychus, many full stomachs

100% B. anonyrhus, many full stomachs

100% B. anonychus

100% squids (B. anonychus?)

squid (Chiroteuthis?)

heteropods, squids, amphipods

almost all squid (B. anonychus?)

squids, fishes

heteropods, squids, isopod

squid beaks

hyperiid amphipods, squids, fishes (Alepisaurus)

large squid, beaks

large cranchiid squid

Alloposid octopod

pteropods (Clio and Umacina) , amphipods, polychaetes

euphausiids. crustaceans

squids, heteropods, fish

squids, pteropods, heteropods, isopods, hyperiids

salps

salps, gelatinous zooplankton

squid beaks, 1 Anotopterus?

squid beaks, 1 eonatopsis borealis

saury? squid beaks

squids, crustaceans
cate that the incidental catch of salmonids has been

low within the regulated squid fishing area but could

increase north of the northern boundary or during

late summer or fall months (Burgner et- al. 1982;

Burgner and Meyer 1983; Walker and Burgner 1985;

Harris and Kautsky 1987). Ogura and Takagi (1987)

reported that salmonids were caught in water temperatures between 1° and 15° C while squid were

caught between 9° and 15° C, but they concluded

that the catches of salmonids were negligible in waters of 14° C, which was the lower limit of optimal

water temperature for squid.

Common Epipelagic Fishes _

This section briefly reviews the distribution, migration, and food habits of common epipelagic fishes of

the transition region. Catches of blue shark, Pacific

saury, Pacific pomfret and albacore are often large in

gill nets fished in the mid-Pacific transition region.

Pacific mackerel and jack mackerel are also taken

closer to the coasts in the western or eastern Pacific.

Hart (1973) and Brodeur (1988) provided more detailed descriptions of the biology of these species.

Blue Shark (Prionace glauca)

The blue shark is found in warm temperate, subtropical and tropical oceans of the world (Hart 1973). It is

common in the North Pacific between 20° and 50° N

and makes pronounced migrations northward into

subarctic waters during the summer and southward

into the transitional zone and subtropical waters during the winter (Strasburg 1958; Neave and Hanavan

1960; Larkins 1964; Parin 1968). Most of the blue

shark caught north of 40° N are females which apparently migrate north to feed. Females with

embryos were only found south of 35° N (Strasburg

1958) .

Food of blue shark caught in gill nets in the Gulf

of Alaska consisted of salmon, pomfret, saury,

lanternfishes, and daggertooths (Anotopterus

pharao) , squids, shrimps and salps; chunks of salmon

were apparently taken from salmon caught in the

gill net (LeBrasseur 1964). Strasburg (1958) found

mostly fishes and cephalopods in the stomachs of

blue shark caught on longlines and concluded that

they are opportunistic feeders. Blue shark examined

from Oshoro Maru gillnet catches contained mainly

remains of large cephalopods, including beaks. Exclusive of fresh fish that were common in gillnet

catches, only one macrourid fish was identified

(Table 3).

Pearcy: Biology of the Transition Region 49

Pacific Saury (Cololabis saira)

The Pacific saury is widely distributed in epipelagic

waters of the North Pacific between 19° Nand 58° N

(Kobayashi et al. 1968; Odate 1977). Three centers

of distribution are reported in the North Pacific

(eastern, central, and western) which may represent

genetically distinct stocks (Inoue and Hughes 1971;

Macy et al. 1978). Seasonal migrations related to

feeding and spawning occur in all three subpopulations. During the summer, saury are common in

subarctic waters and even enter the Bering Sea

(Kobayashi et aI. 1968; Trumble 1973; Macy et aI.

1978). According to Odate (1977) and Shimazaki

(1986), large saury migrate the farthest north across

the subarctic boundary during the summer to feed.

Spawning apparently occurs after migration south of

the subarctic boundary in the western Pacific (Sato

1981). Saury feed on zooplankton, largely copepods,

euphausiids, and amphipods (Hotta and Odate

1956).

Pacific Pomfret (Bramajaponica)

Pacific pomfret are abundant in the North Pacific

Ocean. They perform pronounced northerly migrations from the transition zone across the subarctic

boundary into subarctic waters during the summer

and retreat to the southern part of their range into

subtropical/transition zone waters in the winter

(Neave and Hanavan 1960; Machidori and

Nakamura 1971; Shimazaki and Nakamura 1981;

Shimazaki 1989). The northward movements clearly

correspond to the latitudinal advance of the 9-10° C

isotherm (Trumble 1973; Shimazaki 1989) but distributions may also be influenced by the vertical

thermohaline structure (Shimazaki and Nakamura

1981). Large pomfret migrate the farthest north during the summer, whereas small pomfret are most

common in the vicinity of the subarctic boundary.

During the summer pomfret increase their body

weight and fatness substantially (Shimazaki and

Nakamura 1981).

Gonatid squids and fishes (including saury, sardine and Atka mackerel, Pleurogrammus monopterygius) compose most of the stomach contents of

pomfret, but crustaceans may be important prey for

small pomfret (Pinckard 1957; Machidori and

Nakamura 1971; Wada and Murata 1985; Kubodera

and Shimazaki 1989). Sometimes the pteropod C{io

pyramidata is important (C. B. Miller, College of

Oceanography, Oregon State Univ., Corvallis, OR

97331 pers. commun. June 1989). Stomachs of
50 NOAA Technical Report #105

pomfret taken by the Oshoro Maru In the Gulf of

Alaska frequently contained only squid (mainly

Berryteuthis anonychus) (Table 3).

Albacore (Thunnus alalunga)

Albacore is cosmopolitan in warm, temperate waters

of the world and common in the transition region of

the North Pacific. Laurs and Lynn (1977) recognized

two populations of albacore in the North Pacific: a

northern group that undertakes trans-Pacific migrations from waters offJapan to North America, usually

north of 40° N, and migrates into the transitional

and dilute domains of the Northeast Pacific Ocean,

and a southern group that is found off California

during the summer that has migrational patterns differing from the northern group. Both types of

albacore spawn in subtropical waters presumably in

the western Pacific (Laurs and Lynn 1977; Sund et al.

1981; Laurs, 1991).

The food of albacore consists mainly of fishes and

squids. Saury, lanternfishes, lancetfish (Alepisaurus),

sergestid shrimps, and heteropods are SOmetimes important prey (Iverson 1962, Iversen 1971; Pearcy

1973; Table 3).

Pacific Mackerel (Scomberjaponicus)

Pacific mackerel or chub mackerel is a transitionzone species that inhabits warmer water of the

eastern and western boundary currents but migrates

into the Gulf of Alaska and waters south of the Aleutians during the summer (Larkins 1964; Macy et al.

1978; Mishima 1981; Brodeur 1988). During warm

years, such as the strong 1982-1983 EI Nino years,

Pacific mackerel were abundant from Oregon into

British Columbia (Ashton et al. 1985; Brodeur and

Pearcy 1986; Pearcy and Schoener 1987).

Spawning occurs during the winter mainly close to

shore off southern and Baja California and offJapan

(Kashahara 1961; Macy et al. 1978; Belyayev 1984).

Pacific mackerel feed on a variety of prey including

copepods, euphausiids, shrimps, larval and juvenile

fishes and cephalopods (Fry 1936; Fitch 1956: Hart

1973; Ashton et al. 1985; Brodeur et al. 1987).

Jack Mackerel (Trachurus symmetricus)

Jack mackerel are a transition-zone species found

along the west coast of North America but not in the

western Pacific. They are sometimes common in

gillnet catches (Jamieson and Heritage 1987) or in

purse seine catches (Brodeur and Pearcy 1986;

Pearcy and Schoener 1987) in the northeastern Pacific. They migrate north as water temperatures

increase during the summer and range into the Gulf

of Alaska (Neave and Hanavan 1960; Brodeur and

Pearcy 1986). They occur hundreds of miles offshore.

Jack mackerel form large schools at the surface, occasionally mixed with Pacific mackerel (Trumble 1973;

Brodeur 1988).

Eggs and larvae of jack mackerel are found as far

as 1,100 miles offshore but have not been reported in

the central Pacific (Macy et al. 1978).

Jack mackerel feed on zooplankton and small nekton. Copepods, euphausiids and pteropods were the

main prey off California (Carlisle 1971), and euphausiids were the main prey off Oregon, followed by

pteropods (Brodeur et al. 1987).

The Pelagic Food Web _

Several interesting trends can be derived' from the

observations on food habits of epipelagic nekton

caught in gill nets along long. 155° W from lat. 44° N

to 36°30' N (Table 3). In both the southern portion

of the subarctic domain proper and the northern

portion of the transitional domain, the gonatid squid

Berryteuthis anonychus was the major prey species for

pomfret, flying squid, and albacore. Pomfret preyed

almost exclusively on B. anonychus at stations from

48° N to 44° N. Coho, sockeye, and pink salmon, as

well as steelhead, caught in gill nets at 48° and 4.7° N

also preyed intensively on this squid (Pearcy et al.

1988). Gonatid squids appear to be a key prey for

epipelagic nekton in the Subarctic Current region of

the Gulf of Alaska during the summer.

The diets of different species show similar changes

with latitude. For example, pteropods and

heteropods were well represented in the stomach

contents of flying squid, pomfret, and albacore at

41 ° N. Crustaceans (hyperiid, amphipods, and isopods) appeared more frequently in the diets of these

species caught at the lower than at the higher latitude stations. In general, the stomach fullness of the

pomfret and flying squid was highest at northern stations where squids were the principal prey.

Gonatid squid apparently are not a major prey for

flying squid in the western Pacific. Naito et al. (1977)

reported that small fishes (the lanternfish Diaphus

roerulens, and Engraulis japonica) and squids

(Watasenia scintillans and small O. bartrami) occurred

most frequently in the stomachs of large flying squid

off northeastern Japan. Small flying squid consumed

large zooplankton such as euphausiids, large copepods, and amphipods.

A food web diagram for common epipelagic nekton of the transition region (Fig. 4) is based on the

previous references on the food habitats of common
Pearcy: Biology of the Transition Region 51

Figure 4

Generalized food web for common nekton of the

transition region, eastern and western Pacific

combined, except for dashed lines that pertain

to eastern Pacific.

fishes, the results of stomach analyses aboard the

Oshoro Maru (Table 2), and general reviews of the

diet of epipelagic fishes (Kohno 1983; Shimazaki

1986; Brodeur et al. 1987) and squids (Naito et al.

1977). A generalized food web for the subarctic and

transitional zones is also given by Brodeur (1988).

Assuming that most of the primary production is

consumed by microzooplankton which in turn is ingested by crustaceans and pteropods, the main

species caught in gill nets in the transition region,

such as large flying squid, pomfret and albacore,

probably occupy trophic levelS. Large blue shark,

tunas, and billfishes are the next higher trophic level,

whereas saury and small squids, myctophids and sardine are about trophic level 4. Most large fishes and

squids are opportunistic feeders and the potential for

competition exists between common species such as

pomfret, flying squid, and albacore that consume

similar prey (Table 3). Small gonatid squids appear

to be particularly important in the northern part of

the transition domain in the eastern Pacific during

the summer. Sardine and myctophids appear to be

more important as forage species in the western Pacific than in the eastern Pacific.

Acknowledgments _

1 thank C.B. Miller, R.D. Brodeur, J.A. McGowan and

an anonymous reviewer for helpful comments on the

manuscript. JIMAR Qoint Institute for Marine and

Atmospheric Research), the Northwest and Alaska

Fisheries Center, and the Northwest Fisheries Science

Center helped to support this research.

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Parin, N.V.

1968. Ichthyofauna of the epipelagic zone. Akad. Nauk

SSSR. Transl. Israel Program Sci. Transl., 1970,206 p.

Park, K.

1967. Chemical features of the subarctic boundary near

1700 W. J. Fish. Res. Bd. Can. 24:899-908.

Parslow, J.S.

1981. Phytoplankton-zooplankton interactions: data analysis

and modeling (with particular reference to Ocean Station P

[50 0 N.145° W] and controlled ecosystem experiments).

Ph.D. diss. Univ. British Columbia, 400 p.

Parsons, TR., and C.M. Lalli.

1988. Comparative oceanic ecology of the plankton communities of the subarctic Atlantic and Pacific Oceans.

Oceanogr. Mar. BioI. Ann. Rev. 26:317-359.

Parsons, TR., and R.J. LeBrasseur.

1968. A discussion of some critical indices of primary and

secondary production for large-scale ocean surveys. Calif.

Coop. Oceanic Fish. Invest. Rep. 12:54-63.

Pearcy, W.G.

1973. Albacore oceanography off Oregon-1970. Fish. Bull.

U.S. 71:489-504.

1977. Variations in abundance of sound scattering by animals off Oregon, in Oceanic sound scattering prediction

(N.R. Andersen and B.J. Zahuranec eds.), p. 647-666. Plenum Press.

Pearcy, W.G., and A. Schoener.

1987. Changes in the marine biota coincident with the

1982-1983 EI Niiio in the northeastern subarctic Pacific

Ocean. J. Geophys. Res. 92, o. CI3:14,417-14,428.

Pearcy, W.G., R.D. Brodeur, J.M. Shenker, W.W. Smoker, and

Y. Endo.

1988. Food habits of Pacific salmon and steelhead trout,

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Gulf of Alaska, 1980-1985. Bull. Ocean Res. Instit. Univ.

Tokyo No. 26 (Pt.II.):29-78.

Pinckard, W.F.

1957. Pomfret off the British Columbia coast. Fish. Res.

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Powell, D.E.

1957. North Pacific albacore tuna exploration by the M/V

JohnN. Cobb-1956. Comm. Fish Rev. 19(6):1-9.

Reid,J.L.Jr.

1962. On circulation, phosphate-phosphorous content and

zooplankton volumes in the upper part of the Pacific

Ocean. Limnol. Oceanogr. 7:287-306.
Reid, J.L. Jr., E. Brinton, A. Fleminger, E.L. Venrick, and J.A.

McGowan.

1978. Ocean circulation and marine life. In Advances in

Oceanography (H. Charnock and G. Deacon eds.J, p. 65-

130. Plenum Publ. Corp.

Risher, R.L.

1988. 1987. Flying squid research cruise report R/V Hokuho

Maru, unpubl.

Robinson, S.M.C., and G.S.Jamieson.

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25 p.

Roden, G.I.

1991. Subarctic-subtropical transition zone of the North Pacific: large-scale aspects and mesoscale structure. In Biology, oceanography, and fisheries of the North Pacific transition zone and subarctic frontal zone: papers from the North

Pacific transition zone workshop; 9-1 I May 1988,

Honolulu, HI (l.A. Wetherall, ed.), p. 1-38. NOAA Tech.

Rep. NMFS 105.

Roden, G.1.

1970. Aspects of the mid-Pacific Transition Zone. J. Geophys. Res. 75:1097-1109.

1975. On North Pacific temperature, salinity, sound velocity

and density fronts and then relation to wind and energy

flux fields. J. Phys. Oceanogr. 5:557-571.

1980. On the subtropical frontal zone north of Hawaii during the winter. J. Phys. Oceanogr. 10:342-362.

Sato, M.

1981. Some considerations on the "north to sou th turning

migration" mechanism of the Pacific saury, Cololabis saira

(Brevoort). Res. Inst. orth Pac. Fish., Fac. Fish. Hokkaido

Univ. Spec. Vol., 73-78.

Shimazaki, K.

1986. Distribution of the pelagic fish community around the

subarctic boundary in the North Pacific Ocean. Int. North

Pac. Fish. Comm. Bull. 47:247-264.

1989. Ecological studies of the pomfret (Brama japonica) in

the North Pacific Ocean. Can. Spec. Publ. Fish. Aquat. Sci.

108:195-205.

Shimazaki, K., and S. Nakamura.

1981. Ecological studies of the pomfret (Brama japonica

Hilgendorf). I. the seasonal distributional pattern and ecological considerations. Res. Inst. North Pac. Fish., Fac.

Fish. Hokkaido Univ., Spec. Vol., 91-103.

Sloan, N.A.

1984. Canadian-Japanese experimental fishery for oceanic

squid off British Columbia, summer 1983. Can. Industry

Rep. Fish. Aquat. Sci., No. 152; 42 p.

Strasburg, D.W.

1958. Distribution, abundance, and food habits of pelagic

sharks in the central Pacific Ocean. U.S. Fish. Wildl. Servo

Fish. Bull. 58:335-361.

Sund, P.N., M. Blackburn, and F. Williams.

1981. Tunas and their environment in the Pacific Ocean: a

review. Oceanogr. Mar. BioI. Ann. Rev. 19:443-512.

Taniguchi, A.

1981. Plankton productivities in the Pacific subarctic boundary zone: food conditions of the migrating pelagic fishes.

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35.

Trumble, RJ.

1973. Distribution, relative abundance and general biology

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North Pacific Ocean. M.S. thesis, Univ. Washington, Seattle, 178 p.

Pearcy: Biology of the Transition Region 55

Venrick, E.L.

1971. Recurrent groups of diatom species in the North

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Venrick, E.L.,J.A. McGowan and A.W. Mantyla.

1973. Deep maxima of photosynthetic chlorophyll in the

oceanic Pacific. Fish. Bull., U.S. 71:41-52.

Wada, T., and M. Murata.

1985. Distribution and biological characteristics of pomfret,

Brama japonica Hilgendorf, in the Pacific Ocean off northern Japan. Bull. Hokkaido Reg. Fish. Res. Lab., No. 50,

1-12.

Walker, R.V., and R.L. Burgner.

1985. Surface temperatures and distribution of steel head

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drift gillnel fishery for flying squid (Ommastrephes bartrami).

Document submitted to the annual meeting of the INPFC,

Tokyo, Japan, ovember 1985, 12 p. Fish. Res. Inst. FRIUW-8505, Univ. Washington, Seattle.

Willis,J.M.

1984. Mesopelagic fish faunal regions of the Northeast

Pacific. BioI. Oceanogr. 3:167-185.

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1988. Zoogeography of midwater fishes in the subarctic

Pacific. Bull. Ocean Res. Inst. Univ. Tokyo No. 26 (Pt.

II):79-142.

ELIZABETH HACKER SINCLAIR

ABSTRACT

College of Oceanography

Oregon State University

Corvallis, Oregon 97331

1977b; Murata 1987). However, no genetic studies of

stock discreteness have been conducted. Low densities reportedly occur between 165° E and 170° E

longitude relative to high densities west of 165° E

and east of 170° E, suggesting little intermixing, and

reproductive isolation of at least two stocks (Araya

1983). Maximum body size and rate of growth may

vary between the two regions and Ommastrephes

bartrami may reach a larger size at sexual maturity in

the central and northeast Pacific than in the northwest region (Araya 1983; Murata 1987). Features that

separate groups in these two regions include the Emperor Seamounts (33°30' N to 45° N 170° E), and the

shifting southern extent of the transition zone

(Roden 1991).

Two to four size classes of Ommastrephes bartrami

have been described from the North Pacific, based

on monthly mantle length distributions and maturity

states (Murata and Ishii 1977; Murakami et al. 1981;

Murata 1987). The different size classes and maturity

states may be due to extended or disjunct hatching

periods in winter Oanuary-February) and spring

(April-May) (Murata and Ishii 1977; Naito et al.

1977a; Araya 1983). Disjunct spawning periods are

common among the Ommastrephidae (Roper et al.

1984), giving rise to two or more growth groups. But,

lifespan is variable between the different growth

groups of Ommastrephes bartrami, and the possibility of

distinct populations (Murata 1987) has not been

eliminated.

This review is the first attempt to compile published and unpublished data from

American, Canadian, Japanese, Korean, and Taiwanese researchers on the life history of

Ommastrephes bartrami. Much of the available information in the literature is conflicting,

in part because of the complex and dynamic nature of the biology and distribution of

this species. Given the intense pressure on Ommastrephes bartrami and other squid species

by high seas gillnet fisheries, timely analysis and comprehensive review of forthcoming

data is critical to the conservation of the North Pacific ecosystem.

Ommastrephes bartrami (family Ommastrephidae) is a

transition zone species occurring worldwide in oceanic, near surface waters of temperate and

subtropical regions (Fig. 1). Distribution in the

North Pacific is from Japan to North America, between approximately 25° Nand 50° N latitude

(Naito et al. 1977a; Murakami et al. 1981; Murata

1990). The northern limit of distribution in the

northwest and central Pacific, is the transitional domain of the subarctic water mass (Kubodera et al.

1983; for description of oceanographic terms, see Favorite et al. 1976, and Roden or Pearcy 1991). In the

northeast Pacific, Ommastrephes bartrami has been reported from the broad transitional domain off

British Columbia as far north as 53° N Oamieson

and Heritage 1987), however, no collections have

been reported from the Gulf of Alaska. Ommastrephes

bartrami does not occur in the Bering Sea (Kubodera

et al. 1983).

Review of the Biology and Distribution of the Neon Flying Squid

(Ommastrephes bartrami) in the North Pacific Ocean

General Distribution _

Stocks in the North Pacific

Based on indications of separate spawning grounds

and on distributional patterns from jigging and tagging surveys, more than one stock of Ommastrephes

bartrami may exist in the North Pacific (Naito et al.

57
58 NOAA Technical Report #105

F;01~1111111~llilllll\

80° N

60°

40°

30°

20°

10°

0°

10°

20° S

30·

40°

50°

60°

70·

40° E 60° 80° 100° 120° 140° 160° 180° 160° W 140° 120° 10Co 80° 60° 40° 20° 0° 20° E

Figure 1

Worldwide distribution of Ommas/rephfs bar/rami. (Adapted from Roper et al. 1984.)

The long or disjunct spawning period of

Ommastrephes bartrami produces what has been generally interpreted as large and small cohorts, or winter

and spring brood stock (Murakami et al. 1981), or

early and late migrants (Murata and Ishii 1977). The

described growth categories are extra large sized

(LL), large (L), small (S), and very small (SS)

(Murata and Ishii 1977; Murakami et al. 1981). The

ranges in dorsal mantle lengths given below are

based on squid caught both by jigging and gillnetting

(research and commercial size nets):

LL This group consists of females only. They are

though t to live for two years and mature and

spawn in their second winter in January and February. The LL females reach a maximum dorsal

mantle length (DML) of 48-53 cm (Tung 1981;

Kubodera et al. 1983).
Sinclair: Biology and Distribution of the Neon Flying Squid 59

L This group, also known as the winter brood

stock, consists of males and females that live for

one year. Breeding takes place in October

through December, and females spawn in January and February. The dorsal mantle length

range for a combined sample of males and females collected in October, 1984 was 31-40 em

(Murata and Shingu 1985).

S This group, also known as the spring brood stock,

consists of both males and females. Lifespan is one

year with breeding and spawning in March and

April. The DML range for a combined sample of

males and females collected in October, 1984, was

25-31 em (Murata and Shingu 1985).

SS This group is composed of both males and females. Males first appear in the fishing ground in

autumn and winter and are mature by winter and

spring at a mantle length of 30 em. Males die

after mating in spring and are probably part of

the S group. The S5 females do not mature their

first winter and probably correspond to the LL

type. The maximum mantle length recorded for

a combined sample of males and females of the

SS group in October, 1984, was 25 em (Murata

and Shingu 1985).

Length frequencies described for the four growth

groups are sometimes inconsistent between researchers and years sampled, owing in part to variable

sampling methods and the definition of size categories. However, comparison of early (Naito et al.

1977a; Murata and Ishii 1977) and more recent reports (Araya 1983; Murata et al. 1988; Murata 1990)

50·N

i~ 40·

0..;.0----0 North Paclf.c ~rrent

..: • •• 0

',;~~o 0 •• \ ° • • Midway Is. • 30· • "

•• .. • • • Haw.iiBn Is. ·0

20· ..I>

140·E 150· 160· 17d' 180· 170· 160· 150· 140· 130·W

Migration: o northward migration (Jun.-Sept.)

@ southward migration (Oct.-Dec.)

• spawning migration (Jan.-May)

Catch by jigging per night:

o : 1 ton> o : 1-2.9 tons o :3 tons

Figure 2

Distribution of Ommastrephes bartrami in the North Pacific from June, 1978 to February, 1980. A schematic diagram

ofsurface currents after Dodimead et al. (1963). (Reprinted from Murakami et al. 1981.)
60 NOAA Technical Report #105

of length frequencies, suggests that the sizes of

O. bartrami caught in both the jigging and gillnet

fishery are decreasing.

Ommastrephes bartrami, like other Ommastrephidae, is a thick-mantled, powerful swimmer and

undergoes extensive seasonal migrations. Seasonal

movements have been best described for this squid

in the northwest Pacific (west of 165° E), but limited

data from the central and northeast Pacific are available as well.

Seasonal Distribution _

Spring migrants from subtropical waters (south of

34° N) reach the region of the subarctic boundary

and transitional domain in early summer and return

to subtropical waters by the following spring. Seasonal movements have been verified by tagging

experiments in the western Pacific (Araya 1983) and

commercial catch data across the Pacific (jerry

Wetherall, NMFS, Honolulu, HI 96822, pers.

commun. May 1988). Araya (1983) reported squid

tagged and recaptured west of 165° E (N=28,

15-39 cm mantle length) averaged a travel speed of

8 km/day during northbound movements May to

August, 1.6 km per day from August to October, and

16 km per day during the southbound phase from

October to December.

Seasonal movements apparently occur in a clockwise fashion in the northwest Pacific. Late spring

through early fall (May through September), flying

squid travel north, then northeast, following the

movement of the Kuroshio Current and its frontal

zone (Murata et al. 1976; Araya 1983; Murata et al.

1983) (Fig. 2). The primary migration route is

thought to be in relation to the flow of the northern

warm-water branches of the Kuroshio Current at longitudes 144°-146° E, 148°-151° E, and 154°-156° E

(Araya 1983). The densest concentration in May and

June in the northwest Pacific is found at 36°-38° N

and 144°-156° E. Northward migrating squid arrive

and concentrate along the subarctic boundary (42°-

43° Nand 173°-175° E) by July. Between July and

November, the densest concentrations are along the

subarctic boundary between 39°-46° N west of 165° E

and between 170° E and 177° W. Commercial and research data show that only large females penetrate

the subarctic boundary to move northward into the

transitional domain (Kubodera et al. 1983), where

they are found at 40° N between 180° and 160° W

and up to 50° N between 165° Wand 130° W, June

through September.

During the fall through early winter (October-December), the population moves south and southwest

ahead of the expanding cold Oyashio Current in the

northwest Pacific (Murata et al. 1976; Araya 1983).

Exact locations, January through May, are unknown,

but northwest, central, and northeast Pacific groups

all spawn in the subtropical region at this time

(Murakami et al. 1981) (Fig. 2). Spawning concentrations in the northwest subtropical Pacific occur at

27°-32° N and west of 156° E, in the region of the

Kuroshio Counter-Current (Murata et al. 1976; Araya

1983). Dense concentrations in early winter have also

been found off the Boso Peninsula in Japan

(35°30/ N 140° E) (Murata et al. 1976). Paralarvae

have been described from the Hawaiian Archipelago,

between the islands of Midway and Oahu (Young and

Hirota 1990).

Ommastrephidae generally produce large numbers

of small eggs which hatch within a few days to a few

weeks (Roper et al. 1984). Spawning behavior, parental investment after spawning, and movement and

growth of eggs and larvae have not been described

for Ommastrephes bartrami. Young-of-the-year in the

western Pacific presumably remain in the hatching

area until they reach juvenile growth stages, at which

time they move north with the Kuroshio extension,

growing to the subadult stage by May-August.

Based on research vessel data (jerry Wetherall,

NMFS, Honolulu, HI 96822, pers. commun. May

1988), similar seasonal patterns of movement occur

at approximately the same time and to the same extent in the central and northeast Pacific. However,

little is known regarding association of these squid

with current systems or winter distribution of the supposed east Pacific stock.

Seasonal Movement Relative to

Temperature and Salinity _

Ommastrephes bartrami occurs in greatest abundance

where surface water temperatures are warmer than

10° C, with highest catches in surface temperatures

of 15°_20° C (Fig. 3) (Murata et al. 1976; Naito et al.

1977b; Kubodera et al. 1983; Murata et al. 1983;

Gong et al. 1985a; Murata 1987). Distribution in summer through early winter is primarily in upper

surface layers at night, and correlates strongly with a

surface layer temperature range of 15°-24° C in July

and August, and 10°_22° C, September through December (Roper et al. 1984). This is confirmed by data

collected from commercial and research vessels between 1983 and 1986 which showed catches of

O. bartrami between 10°-26° C across the Pacific

(Jerry Wetherall, NMFS, Honolulu, HI 96822, pers.

commun. May 1988). Flying squid have been caught

in water as low as 6° C, however, this seems to be

uncharacteristic of the species (Naito et al. 1977b;

Kubodera et al. 1983).
Sinclair: Biology and Distribution of the Neon Flying Squid 61

e

;; 50

C'OO

E

i" 50

t: 8 ..,

C,OO

38·

c=:J Ornmoltrlph"

[IT]]] OnychotOu1hl. borooll,aponlca

_Go"",opsis~

..0-__4--;---:-

48°N

(310) (I'll)

c:::::J Ommgs'rephas

(/$8) lJIIIl Onycho'ou'h1. borooSloponlca

_ Go",,'op." borealis

23 JULY 1981

175°30'[ Figure 3

Vertical features of temperature and

salinity from the surface to 125 m

depth, along with CPUE of Gonatopsis

borealis, Onychoteuthis borealijaponica,

and Ommastrephes bartrami in late July

1979-1981. (Reprinted from

Kubodera et al. 1983.)

The avoidance of water temperatures below 10° C

may restrict Ommas/rephes bartrami to surface layers

above the summer and fall thermocline (less .than

50 m depth) in the transitional domain and along

the subarctic boundary (Kubodera et al. 1983). Salinity. tolerances are less well defined but, the

thermohaline structure of convergence zones formed

by the warm Kuroshio and cold Oyashio Currents, by

the subarctic boundary, and by the boundary between the transitional and subarctic domains may act

as barriers to further northward movement and result in large concentrations of squid along these

boundaries (Kubodera et al. 1983). These boundaries

may be more of a barrier to squid prey than to squid,

and thus serve as a preferred feeding area for

O. bar/rami.

Based on the behavior of other ommastrephids

(Roper et al. 1984), Ommastrephes bartrami probably

undergoes diel vertical migrations. The presence of a

ventral silver stripe that is a distinguishing characteristic of the species suggests a predominantly meso- to

epi-pelagic lifestyle. Nighttime distribution, as indicated by gill net catches, extends into the upper 10m

of the water column. Small (<18 cm DML) flying

squid have been observed at the sea surface day and

night (Murata 1988). Adult Ommastrephes bar/rami

may reach depths of 1500 m during the winter and

spring spawning seasons in subtropical waters (Roper
62 NOAA Technical Report #105

et al. 1984), but are more typically in the top 100 m

of the water column during the summer and fall

feeding period in northern waters (Murata 1988).

Seasonal Movement

Relative to Sex and Size _

Patterns of seasonal migration in the northwest Pacific are different between sexes, and between large

and small females (Fig. 4). Young-of-the-year squid

are in juvenile growth stages during the northward

migration. The oldest of this age group migrate

north first (Naito et al. 1977b; Murakami et al. 1981)

and are generally located farther offshore in the

northwest Pacific than younger individuals

(Murakami et al. 1981; Gong et al. 1985a). Thus,

body size measurements increase predictably with

latitude during the summer (Tung J 981; Murata

1987) (Fig. 5). Large females that penetrate the sub-

. . .. .. . .. '! . . A

oj!. ll> . ... . . ..

.o~.+----1--+--l----+-~~----l2d'

14d' 150· 160· 17d' 180· 170·

·0

160· 150· 14d'

55 size group

13d' 120"w

I3d'E 140· 150· 16d' 170· lad'

.

.,

Ht:iianI1a.

·0

,

17d' 16d' 15d' 14d'

LL size group

13d' 120·W

Month: o :June-Sept. !::>: October-Nov. \7: December-Feb.

Percentage of catch individuals at each station:

MV: 0% el>Y: 10%> el>'W: 10-19.9% .l>~ : 20-29.9%

el'.W : 30-39.9% .11W : 4049.9% ._~W: 50%
Sinclair: Biology and Distribution of the Neon Flying Squid 63

Z 39.5-

l42.5

1415[

.. 68

83

00

29!i!

o 5111 39

219

152

38 15 20 2S 30 3S .a 45 SO

Mant Ie length in em

S30 175'E

5110 1820 ~20 299~

10 :> f' 0 .... 5112 94

"""T2I

415

15 20 2S 30 3S 40 45 .so Mant Ie length in em

5122 16SoE

JR

576

t 208

205

39

30 3S 40 4S .so Manti. 'ength in em

180° S40

>- 519 30

'520

10

.... 01--------------

1 , I , , , •

30 35 40

Mantle length in em

25

t ,

20

I ,

'5

July

August

55 5 L LL

---""',,

Septeme r -----'-',-,......"

Figure 4 (facing page)

Distribution of 55 and LL mantle-size groups of

Ommastrephes bartrami expressed as percentage of catch

at stations in the North Pacific Ocean from June, 1978

to February, 1980. 0 = June-September; 6 = OctoberNovember; V = December-February. (Reprinted from

Murakami et aI. 1981.)

Figure 5

Classification of the four size groups of flying squid

based on monthly mantle length ranges. 55 = very

small; 5 = small; L = large; LL = very large. (Reprinted from Murata 1987.)

arctic boundary into the transitional domain are less

abundant in both inshore waters ofJapan (Murakami

et al. 1981) and in Japanese waters of the transi tional

domain than they are farther east (Murakami et al.

1981; Murata 1987). This distribution may be related

to the narrowness of the transitional domain (Favorite et al. 1976) in the western Pacific relative to the

central and eastern zones.

Catches of both immature males and females

throughout the orth Pacific are typically highest
64 NOAA Technical Report #105

along the subarctic boundary (Kubodera et al. 1983).

The most densely concentrated shoals are found in

the western Pacific boundary region (Gong et al.

1985a). High concentrations of squid at the subarctic

boundary of the western Pacific may be related to the

steeper horizontal gradients of temperature and salinity in this area than in the subarctic boundary

region of the eastern Pacific (Gong et al. 1985a). Immature males and females (15-25 cm DML) are

caught in equal numbers at the subarctic boundary of

the western Pacific in July (Kubodera et al. 1983).

Catches within the transitional domain of the Subarctic Water Mass are high during late summer.

Kubodera et al. (1983) and Murata (1987) reported

entirely large (LL, 34-48 cm DML) immature females

north of the subarctic boundary (denoted by 34%0 at

the surface). Thus, both the average size and proportion of female flying squid increase with latitude

(Murata and Ishii 1977). Ommastrephes bartrami are

not caught north of the transitional domain in the

northwest or central Pacific (Kubodera et al. 1983).

Males begin southerly migrations first (Naito et al.

1977b; Murakami et al. 1981; Tung 1981). Males mature before females (Murata and Ishii 1977;

Murakami et al. 1981) and southbound migration is

triggered by the onset of sexual maturity (Murata et

al. 1976). Large females lead or overtake the small

females in northerly migrations in the northwest and

central Pacific (Murata 1987) and overtake both small

females and males in their southward movement

(Tung 1981; Gong et al. 1985a).

In summary, males migrate north, mature, return

to sUbtropical waters, copulate, then die in their first

winter and spring. With the exception of the LL and

SS type females, all individuals are thought to migrate, mature, and spawn after one year. The SS

females presumably make an abbreviated, round trip

migration in their first year, pass a full year without

spawning, and grow to become the very large females

that migrate farthest north in their second year, when

they mature, return south, copulate, then die after

spawning in their second winter and spring. Perhaps

the SS females skip the northward migration altogether their first spring, and travel north only during

their second spring as LL females. The northern migration is a time of feeding and growth. The southern

migration is initiated with the onset of sexual maturity and punctuated by spawning.

Biology and Growth _

The average body size of Ommastrephes bartrami caught

in jigs and giHnets across the Pacific is about 27 cm,

within a range of 14-50 cm. Maximum mantle length

reported for females is 53 cm and over 2 kg body

weight. Males reach a maximum length of 41 cm and

l+kg body weight (Tung 1981).

Body weight to mantle length relationship, according to Tung (1981), is

BW = 6.16335 X 10-6 MV2716

(R = 0.9884; n=247) (1)

while body weight to mantle length relationship, according to Murata and Ishii (1977), and Gong et al.

(1985b), is

BW = 1.2799 X 10-5 MV1437. (2)

Estimated growth curves for flying squid in the

northwest Pacific are sigmoid, and the total growth

attained in any given month varies between groups

of large and small body size (Murata and Ishii 1977)

(Fig. 5). Preliminary tagging and capture studies indicate highly variable growth rates (Araya 1983).

Growth rates are intensive during the spring and

summer period of northward migration and decrease

as sexual maturity is approached in late fall and winter (Murata and Ishii 1977). In June-October and

October-December, growth of the early migrants

(males and females combined) is 3-4 cm/mo and

1 cm/mo, respectively. Growth of the small-sized

group (males and females combined) is 2-3 cm/mo

throughout this period. In the central Pacific, Tung

(1981) reported rates of 1 cm/mo growth for early

migrant males in August through October. Late migrant males from late July to mid-September

averaged 3 cm/mo. Growth rates of the late migrating females appears similar to the males in July and

August, although the trends are less clear owing to

combined measurements of large and small sized females (Tung 1981; Araya 1983). December through

February, growth of the large group slows to

0.5-1 em/mo, while the small size group averages

1-2 cm/mo (Murata and Ishii 1977).

Wide deviations from these median growth rates

(up to 8 em/month) have been documented for

some individuals tagged in May through July and recaptured July to October (Araya 1983). The rapid

rate of growth may be correlated with population size

or density, or environmental conditions, particularly

temperature and food availability.

Size at Sexual Maturity _

In the northwest Pacific, males show first signs of

spermatogenesis at 28.5 cm DML and 670 g body

weigh t (Tung 1981). Almost all northwest Pacific
Sinclair: Biology and Distribution of the Neon Flying Squid 65

males examined are fully mature by November or December, at a mantle length of 32.5 cm, testis weight

of 15-20 g, and approximate body weight of 1 kg

(Murata and Ishii 1977; Tung 1981). Females with

spermatophores in January-February are 40 cm DML

and have 20-50 g ovary weights (Murata and Ishii

1977). Females are slower to mature and larger at

maturity than males in all Pacific groups (Murata and

Ishii 1977).

In a 1986 study conducted June through September across the Pacific, using commercal and scientific

gillnets and jigs, Murata (1987) reported that nearly

all females caught were immature and unmated, and

nearly all males were immature and lacked spermatophores. Individuals caught in commercial salmon

gillnets (110-115 mm mesh) were primarily female

(Murata 1987).

Food Habits _

Stomach samples examined from Ommastrephes

bartrami collected by gillnet in the northwest Pacific,

south of the subarctic boundary, primarily contained

fishes and secondarily squids (Naito et al. 1977a;

Tung 1981). Planktonic crustaceans (primarily

Euphausiacea and Parathemisto sp.) were present in

low numbers (Tung 1981) and found in greatest proportions in immature squid (Naito et al. 1977a).

Identified fishes include lanternfishes, anchovy, sardines, mackerel larvae, and saury. Squids eaten were

Watasenia scintillans, Onychoteuthis borealijaponica, and

Ommastrephes bartrami (Naito et al. 1977a).

Pearcy (1991) found remains of fishes, squids,

other mollusks, and crustaceans in the stomachs of

O. bartrami caught in gillnets along 155° W from

36° N 30' and 44° N in the eastern Pacific in July,

1985. Diet was composed almost exclusively of

Berryteuthis anonychus at 44° N.

Other Squids _

Gillnet and jigging research surveys indicate the frequency of occurrence of three other squid species

(eonatopsis borealis, Berryteuthis magister, and

Onychoteuthis borealijaponica) within the North Pacific

driftnet fishing zone is higher than that of

Ommastrephes bartrami (Kubodera et al. 1983). However, O. bartrami forms dense concentrations

compared to the other species, and as a result, the

total numbers of flying squid caught in gill nets is

higher (Kubodera et al. 1983). Naito et al. (1977b)

reported that in gill nets with mesh sizes ranging

from 24-157 mm, the peak catch for all four species

occurred in 48 mm mesh. Murata (1987) reported

that within a range of 33-157 mm mesh, maximum

catches of Ommastrephes bartrami were in 48 mm and

63 mm mesh, while maximum numbers of

Onychoteuthis borealijaponica and e. borealis w-ere

caught in 37 mm and 55 mm mesh, and in 55-82 mm

mesh respectively. Summer research vessel sllrveys

conducted in the subarctic domain, transition zone,

and south of the subarctic boundary found

Ommastrephes bartrami to be the most abundant squid

(46% catch), Onychoteuthis borealijaponica the second

most abundant, and eonatopsis borealis the third most

abundant, however the frequency of occurrence of

the three species was reversed (Kubodera et al.

1983). Of the four species, Ommastrephes bartrami attains the largest maximum mantle length (Naito et

al. 1977a).

Although there is distributional overlap among the

four species, different temperature tolerances segregate them spatially (Naito et al. 1977b; Kubodera et

al. 1983) (Fig. 3). Onychoteuthis borealijaponica is distributed in subtropical and subarctic waters,

primarily in temperatures of 11 °_15° C and depths of

20-30 m. eonatopsis borealis is considered a subarctic

species and is found most abundantly at 5°_10° C, at

depths greater than 40 m. Berryteuthis magister is distributed in the subarctic domain and appears to

overlap least with Ommastrephes bartrami.

Three of the four squid species frequently caught

in the gill nets are known to be important prey of

marine mammals (Fiscus et al. 1964; Kajimura 1984;

Sinclair 1988). Sperm whales reportedly prey on

adult flying squid (Okutani 1977), while juvenile

O. bartrami may be an important resource for other

toothed cetaceans, seabirds, and large predatory

fishes such as tunas. There is justifiable concern

about the degree of incidental take of non-target

species of marine mammals, birds, and fish but

the most dramatic effect of extensive gillnetting for

O. bartrami may be the indirect impact of the removal

of these and other squids as an important food resource. The number of non-target squid species

caught in the commercial gillnet fishery was not consistently monitored prior to 1990.

Trends in Recruitment Success of

Ommastrephes bartrami

In a review of studies based on data collected from

the northwest Pacific in 1976 through 1979, Araya

(1983) described a trend of decreasing mantle length

beginning in 1977, and considered this the result of

overexploitation of winter hatched or large squid.

From gillnet and jigging research data in the northwest and central Pacific, Murata (1987) reported a
66 NOAA Technical Report #105

decreasing trend in squid densities of large sized individuals betwet"n 1984 and 1986. Fishi!l?: effort in

August and September i~ concentrated within the

transition zone and along the subarctic boundary. An

overemphasis in fishing effort on the large pre-spawning females within the transitional domain could

severely reduce the spawning success of the winter

brood group.

A trend of decreasing catch (CPUE) is apparent

between 1977 and 1979 (Tung 1981; Araya 1983), and

1980-1983 (Gong et a!' 1985b). Increased numbers of

squid were caught during the initial expansion of the

fishing gounds and addition of boats; however catchper-unit-effort values reportedly leveled off or

decreased among Taiwanese vessels by 1979, and the

actual catch by Korean vessels in 1986 (59,000 tons)

was lower than 1979 levels (71,000 tons) (Gong et a!'

1985b; Natl. Fi~h. Administration 1987). Fishing effort among Korean vessels in the northwest and

central Pacific more than doubled between 1980 and

1983, and quadrupled by 1986, yet the catch pn unit

net (96-115 mm mesh) declined sharply from 22 kg!

net in 1980 to 7 kg/net in 1983, 4 kg/net in 1984,

and 3.4 kg/net by 1986 (Gong et a!' 1985b; Nat!' Fish.

Administration 1987).

Efforts to calculate an accurate population size and

status for Ommastrephes bartrami in the North Pacific

are complicated by variable research and commercial

fishing methods, inconsistent availability of catch

records, discrepancies in the interpretation of cohort

sile categories, and a lack of detailed knowledge regarding the biology and distribution of the

population. Despite these complicating factors, a general trend towards a decrease in dorsal mantle length,

body weight, and squid densities since the late 1970's

is indicated by the most conservative estimates.

Strong concern for the status of Onimastrephes bartrami

in the Nurth Pacific has been expressed by scientific

representatives from each nation commerciallv

fishing for this species in the North Pacific (Tung

1981; Gung et a!' 1985b; Murata 1990).

Citations _

Araya, H.

1983. Fishery, biology and stock assessment of Ommaslrephes

barlrami in the North Pacific Ocean. .\fern. Natl. Mus. Vie.

No. 44, 26\)-283.

Favorite, F., AJ Dodimead, and K. Nasu.

1976. Oceanography of the Subarctic Pacific region, 1960-

1971. Int. North Pac. Fish. Comm. Bull. No. 33., 187 p.

Tokyo,Japan.

Fiscus, C.H., G.A. Baines, and F.W. Wilke.

1964. Pelagic fur seal investigations, Alaska waters,

1962. U.S. Fish. and Wildl. Servo Spec. Sci. Rept. Fish. 47!'i,

59 p.

Gong, Y, Y Kim, and S. Kim.

1985a. Distribution and migration of flving squid, Ommaslrel,he~ harlrami (LeSueur), in the North Pacific. Bull. Korean Fish. Soc. 1t\(2):166-179.

Cong. Y,j.Y. Lim, and YH. Hur.

1985b. Study on the abundance of flying squid, Ommastrephes

bartrami (LeSueur) in the North Pacific. Bull. Fish. Res. Dev.

Agency 34:127-132.

Jamieson, G.S., and G.D. Heritage.

1987. Experimental flying squid fishing off British Columbia,

1985 and 1\)86. Can. Ind. Rep. Fish. Aquat. Sci., No. 179,

103 p.

Kajimura, H.

1984. Opportunistic feeding of the northern fur seal,

Callorhinus ursinus, in the Eastf>rn North Pacific Ocean and

Eastern Bering Sea. NOAA Tech. Rep. NMFS SSRF-77\),

49 p.

Kubodera, T.. We.. Pearcy, K. Murakami, T. Kobayashi, j. Nakata,

and S. Mishima.

1983. Distribntion and abundance of squids caught in surface gillncts in the subarctic Pacific, 1977-1981. Mem. Fae.

Fish., Hokkaicto Univ., Vol. 30, No. 1/2, Nov., 1-49.

Murakami, K., Y Watanahe, andJun Nakata.

1981. Growth. distribution and migration of'flying squid

(Omma.<lrephcs bartrami) in the North Pacifie. Tn Pelagic animals and environments around the Subarctic Boundary in

orth Pacific (S. Mishima, ed.), p. 161-179. Res. Inst. of

North Pacific Fish. Fae. of Fish. Hokkaido Univ., Spec. Vol.

1981,215 p.

Murata, M.

1987. Report on the fishing survey on flying squid by drift

gillnetters, Shoyo-Maru, Kuromori-Maru No. 38 and KankiMaru No. 58 in the North Pacific in 1986. Document submitted to the Ann. Mtg. of the Int. North Pae. Fish. Comm.,

Vancouver, Can., 1987 October, 20 p. Fish. Agency of Japan, Tokyo,Japan 100.

Murata, M.

1988. On the flying behavior of neon flying squid

Ommaslrephes bartrami observed in the central and northwestnn North Pacific. Nippon Suisan Gakkaishi

,H(7): 11fi7-1174.

Mur"ta, M.

19\)0. Oceanic resources of squids. Mar. Behav. Physiol.

18:19-71.

Murata, M., and M. Ishii.

1\)77. Sume i.lformation on the ecology uf the oceanic

squid, Ul/lUla5trephes bartrami (LeSueur) and Onychoteuthis

borealijaponicus Okada, in the Pacific Ocean off northeastern

Japan. Bull. Hokkaido Re~. Fi,h. Res. Lab., No. 42. March,

73 p.

I\lurata, M.. and C. ShinKu.

14K5. Report on the fishing survey on flying squid by drift

gillnetters, Oumi-Maru and Kanki-Maru No 58 in the North

Pacific in 1984. Document submitted to the [nt. North

Pae. Fish. Comm., 20 p.

Murata, M., M. Ishii, and H. Araya.

1976. The distribution of the oceanic squicts, Ommastrephes

baTtrami (LeSueur), Onychoteuthis borealijaponiws Okada,

Gonatopsis borealis Sasaki and Todarodes pa,i.firus Steenstrup

in the Pacific Ocean off north'"astern Japan.. Bull.

Hokkaido Reg. Fish. Res. Lab. 41 (Feb.):I-~9.

Murata, M., M. Ishii, and C. Shingu.

1983. Seasonal changes in location and water temperature of

the fishing grounds by jigging fishery toc flying squid,

Ommastnphes bartrami (LeSueur), with some (onsiderations
Sinclair: Biology and Distribution of the Neon Flying Squid 67

on migration and occurrenct> of the fishing ground. Bull.

Hokkaido Reg. Fish. Res. Lab. 4R (Aug):53-77.

Murata, M., Y. Nakamura, S. Kubota, T. Hashiba, and

H. Yamaguchi.

1988. Some biological aspects of neon flying squid

Ommastrephes bartmrni caught by Japanese squicl gilinettt>rs

in tht> North Pacific. Nippon Suisan Cakkaishi 54(9):

1573-1581.

Naito. M., K. Murakami, and T. Kobayashi.

1977a. Growth and food habit of oceanic squids

(Omrnastrephes bartrami, Onychoteuthis borealijapnnir.us,

Benyteuthis magister and Gonatof)sis bOt'talis) in the western

Subarctic Pacific region. In Fisheries biological production

in tht> Subarctic Pacific region, p. 339-351. Res. Inst. of

:\Iorth Pac. Fish. Fac. of Fish. Hokkaido Univ. Spec. Vol.

1977,561 p.

Naito, M., K. Murakami, T. Kobayashi, I . Nakayama, and

J. Ogasawara.

1977b. Distribution and migration of oceanic squids

(Ommastrephes bartrami, Onychoteuthis borealijaponicus,

Ber'yteuthis magister and Gonatopsis borealis) in the western

Subarctic Pacific region. In Fisheries biological production

in the Subarctic Pacific region, p. 321-337. Res. Inst. of

North Pac. Fish. Fac. ofFish. Hokkaido Univ. Spec. Vol. 197,

561 p.

National Fisht>rit>s Administration.

1987. National rt>port on the condition of the Korean trawl

fishery off Alaska and flying squid in the l\'orth

Pacific. Pusan, Rep. of Korea, ovember 1987.

Okutani, T.

1977. Stock assessment of cephalopod resources fished by

Japan. FAO. Fish Tech. Pap. (173),62 p.

Pearcy, W.G.

1991. Biology of tht> transition region. In Biologv, oceanography, and fi,heries of the :\orth Pacific transition zone

and subarctic frontal zone: papers from the :-';onh Pacinc

transition zone workshop; 9-11 May 1988, Honolulu, HI

(J.A. Wctherall, t>d.), p. 39-55. NOA!\ Tech. Rep. MFS

105.

Roden, G.1.

1991. Subarn;c-subtropiLal transition zone of the North Pacific: large-scale aspects and mc,oscale structure. In Biology,

oceanography, and fisheries of the North Pacific transition

zone and suban tic frontal zone: papers from the orth Pacific tramition zone workshop: 9-11 May 1988,

Honolulu, HI (J.A. Wt>therall, ed.), p. 1-38. OAA Tech.

Rep. NMFS 105.

Roper, C.F.E., MJ. Sweeney, and C.E. Nauen.

1984. FAO species catalogue. Vol. 3: Cephalopods of the

world. FAO Fish. Synop. 125,277 p.

Sinclair, E.H.

19t18. Feeding habits of the northt>rn fur seal (Cal/orhinus

uninus) in the Easte, n Bering Sea. MSc. thesis, Oregon

State L'niv., ~4 p.

Tung,I.H.

1~81. On the fisher) and the biology of the squid

Ommastrephes bartmmi (LeSueur), in the Northwest Pacific

Ocean. Rep. of the Inst. of Fish. BioI. of Minis. of Econom.

Affairs and Natl. Taiwan Cniv., Vol. III, No.4, December,

12-37.

Young, R.E., andJ. Himta.

1990. Description of Ommastrephe~ baTtrami (Cephalopoda:

Ommastrephidae) paralarvat' with evidence for spawning in

Hawaiian waters. Pac. Sci., 44 (1):71-80.

North Pacific Albacore Ecology and Oceanography

R. MICHAEL LAURS and RONALD J. LYNN

Southwest Hsheries Science CentPT

National Marine Fisheries Service, NOAA

LaJolia, California 92038

ABSTRACT

Albacore, Thunnus alalunga, is a wide-ranging tuna species occurring between latitudes

10° and 50° N in the North Pacific Ocean. Albacore are a resource which supports

important U.S. and Pacific-nation commerical fisheries and U.S. recreational fisheries.

There is a growing body of evidence that there are two subgroups of albacore that have

different migratory patterns, modal sizes, growth rates, and spawning periods, although

they do not appear to be genetically distinct. Pre-spawning albacore, two to five years old,

are highly migratory and conduct well-defined, trans-Pacific migrations. The spawning

adults are generally confined within the subtropical and tropical zones of the central

North Pacific. Their distribution and migration are markedly influenced by the marine

environmental conditions associated with the waters of the North Pacific transition zone

and its frontal boundaries. The subarctic and subtropical fronts are regions of large

gradients in temperature and salinity which lie in bands across much of the North Pacific. The dynamic processes that maintain these gradients influence the nutrient and

biomass distribution. The major centers of albacore catch are found about these fronts

and within the transition zone waters that lie between them.

Introduction _

Albacore, Thunnus alalunga, is a wide-ranging tuna

species occurring between latitudes 10° and 50° N in

the North Pacific Ocean. There is a general geographical separation by age, spawning adults

occurring mostly in the central waters between latitudes 10° and 20° N and pre-spawning juveniles

making extensive migrations between latitudes 30°

and 50° N. The resource supports important U.S.

commercial and recreational fisheries and several foreign fisheries. The distribution, availability, and

migration of the albacore, as well as its vulnerability

to capture, are markedly influenced by marine environmental conditions. A general review of the

ecology, biology. and fisheries operating on the resource are discussed in this report with emphasis on

the physical oceanographic regimes associated with

the North Pacific transition zone.

North of the subarctic front the waters are cold

and have low salinity. South of the subtropical front

waters are warm and have high salinity. The waters

between these large water masses are transitional in

characteristics and are termed the North Pacific transition zone. The narrow zones where the changes in

properties are abrupt are termed fronts. The schematic representation of ocean fronts presented in

Figure 1 is a simplification of the complex features

found.

Fisheries Harvesting

North Pacific Albacore _

Japan and the United States account for the majority

of the catches of North Pacific albacore, catching approximately 72% and 26%, respectively, from 1965

through 1985. Canada, Korea, and Taiwan each land

about 0.5% to 1% of the total catch. Historically, the

Japanese have had two fisheries that harvest North

Pacific albacore, a pole-and-line surface fishery operating during spring and summer and a longline

subsurface fishery operating during winter. In 1978, a

third Japanese fishery gained importance with the

69
70 NOAA Technical Report #105

40'

20'

50'

30'

60'

120'

············;· .. ·..·· .. ..:·-110·

180' 160' 140'

•. \0': \'41> .s.,..

...Subarctic :Front.

"

160'

160'

Ku~;'Shi~ E~te~s~n

~!:::==~ ;::-...;... .~::::;:= .••:,.:'\:..d~~~·%==?;.~ ...;:;.~~~~~~ :Subtropi~al Front: . .

: ....... ...........t>.

WATER MASS FRONTS

140'

140'

: ; ...................

120'

50'

20'

40'

10'

60'

Figure 1

Schematic representation of the major fronts in the temperate zone of the North Pacific based upon numerous individual

observations (dots). The transition zone lies between the fronts.

dramatic expansion of summer gillnet fishing targeting albacore.

For several decades the pole-and-line (bait boat)

fishery extended from near the coast of Japan eastward to about 1500 E. In the early 1970's, the fishery

was expanded further eastward within the Kuroshio

extension waters to near the dateline. However in the

mid-1980's, it was contracted back to about the same

distribution as earlier. The Japanese longline fishery

is conducted across much of the North Pacific, and

by 1981, gillnet fishing operations had also spread

almost entirely across the North Pacific, mostly within

the North Pacific transition zone waters. The major

centers of catches of the Japanese pole-and-line and

longline fisheries are shown in Figure 2A and the

distribution of the gill net fishery in 1981 in Figure

2B.

The U.S. North Pacific albacore fishery, which began in the early 1900's, uses surface trolling and

pole-and-line fishing gear (Dotson 1980). The fishery

takes place during summer and autumn, and for

many decades has operated in waters within a few

hundred miles of the coast between northern Baja

California, Mexico, and British Columbia, Canada. In

1975, the U.S. troll fleet began a broad westward extension of its operating range (Laurs and Nishimoto

1979). Beginning by the late 1970's, 35 to 50 vessels

would start to fish near the date line in about May,

and progress eastward across the mid-North Pacific in

transition zone waters, ending the fishing season in

autumn in California Current waters off the west

coast of the United States. The main centers of

catches of the U.S. fishery are shown in Figure 2A.

Major geographical variations in the location of

the U.S. coastal fishery occur; during some periods it

is centered in waters off the Pacific Northwest, during others, off southern-central California (Laurs et

al. 1976). These geographical shifts in the location of

the fishery have been linked to variations in largescale .environmental conditions (Clark et al. 1975).

The U.S. albacore industry through the cooperation of American Fishermen's Research Foundation I

and NOAA/NMFS scientists has conducted numerI The American Fishermen's Research Foundation is an albacore

fishing industry non-profit organization funded by assessments

imposed by the industry on landings of albacore caught by U.S.

fishermen. The funds are used for research and education

purposes.
Laurs and Lynn: North Pacific Albacore Ecology and Oceanography 71

.....: ,: : : ;.-1O'

60'

40'

50'

20'

120'

.... ···· ......:.. ···... ·..·....··...j10·

140'

1982';';66' •

U.S. JIG & BAITBOAT

. rzl >100'fish • .........!2Zl.)10,000 fish

160'

180'

180'

, .. .....; I'·

ALBACORE

1969-79

.... JAPAN·ESE· -LONG·LINE·:· '~~t:>"

t·:;:::;:;::l>9.000fish/5°block .

160'

40'

10'

20'

60'

50'

40'

30'

20'

160'

.,

180'

..: ,~ :'" .A-.

180'

. '.. .......l) ....

160' 140'

120'

·......·........·....·.. ·--120·

··•·.... ·....·....·,·-110·

...................... ,·-10'

120'

Figure 2

(A) Major centers of catch for Japanese Pole-and-line, longline, and U.S.jigboat fisheries. The contoured and shaded

regions represent approximately 75% of total annual average for the years indicated for each fishery. The contoured

regions having finer hatching Oapanese baitboat and U.S. jig and baitboat only) indicate areas of highest concentrations of catch. (8) Distribution of the Japanese drift gill-net fishery for orth Pacific albacore in 1981, indicated by

the shaded area.
72 NOAA Technical Report #105

n umbers of a] bacore in the North Pacific.

TOTAL CATCH BY FISHERY There are a number of factors which are associated

30 with the decline in the traditional North Pacific albaU.S. Surface core surface fisheries, but the relative importance of

20 these factors is unknown. Parrish et al. (1989) discuss

trends and status of the fisheries and the North Pa10 cific albacore population.

0

30 55 60 65 70 75 60 85 Stock Structure

20 There is a growing body of evidence (Brock 1943; ,.,

0 Laurs and Lynn 1977; and Laurs and Wetherall 1981)

x 10 that the population of North Pacific albacore is not

I6 as homogeneous as has been assumed (Clemens

::I: 0 1961; Otsu and Uchida 1963). Results from tagging 55 60 65 70 75 80 85 0 20 l- studies suggest that there are two subgroups of albae(

0 Japanese Gillnet core which have different migratory patterns (Laurs 10 1983), modal sizes (Brock 1943; Laurs and Lynn

1977; Laurs and WetheraU 1981), growth rates (Laurs 0 55 60 65 70 75 80 85 and Wetherall 1981), and peak spawning periods 90

80 (Wetherall et al. 1987) . Off the coast of North Japanese 70 America, the boundary between the two subgroups 60 Saitboat

50 appears to be situated near 40° N. Although the pro- 40 posed subgroups are differentiated by geographic 30

20 dissimilarities in biological or fishery statistic criteria, 10 they do not appear to be genetically distinct (Graves 0 55 60 65 70 75 80 85 and Dizon 1989).

YEAR

Figure 3

Annual catches of North Pacific albacore by country and

year for 1955-1983 (from A. Coan, G. Rensink, C. Perrin,

and F. Miller, NMFS SWFSC Adm. Rep. LJ-90-21, "Summary

of the 1989 North and South Pacific albacore fisheries data"

1990).

ous research operations, including a series of exploratory fishing and research operations to evaluate the

feasibility of establishing a winter U.S. longline fishery for albacore in the eastern Pacific (Laurs and

Dotson 1983). The results of the experiments are encouraging, and while a number of fishermen have

expressed an interest in participating in this winter

expansion of the U.S. albacore fishery, none are doing so at this time.

Annual catches of North Pacific albacore by country and gear type are given in Figure 3 for years

1955-1989 and in Table 1 for years 1952-1988. Fishing effort and catch in both the Japanese baitboat

and U.S. surface fisheries have declined beginning in

the early 1980's. In contrast, recent landings and effort in the Japanese longline fishery have been

relatively constant and there has been a rapid development of Asian gillnet fisheries that take large

Migration _

North Pacific albacore are one of the most highly

migratory species of tuna. The exten t and degree of

migration are most expansive in pre-adult ages between about 2 and 5 years. Fish of these ages may

conduct trans-oceanic migrations or migrations

across broad regions of the North Pacific, mostly

within transition zone waters. The migration habits of

the spawning adults, six years of age and older, are

much more limited and are confined mostly within

the subtropical and tropical zones of the central

North Pacific.

Tagging results show that albacore that had been

tagged and released in coastal waters off North

America north of lat. 40° N have a different migration pattern than those tagged south of 40° N. For

example, for fish that had been tagged and released

in the northern area and had been at liberty for up

to 5 years, 28 percent were recovered in the same

general area as release, 54 percent west of long. 180°,

13 percent in coastal waters off North America south

of 40° N and about 5 percent in the central eastern

Pacific or unknown area of recovery (Fig; 4A). In

contrast, for fish that had been tagged and released
Table I

Catches of North Pacific albacore in metric tons by fisheries, 1952-1988 (from N. Bartoo and Y. Watanabe, NMFS SWFSC Adm. Rep., "Report of the eleventh

North Pacific albacore workshop, May 18-19, 1989" 1989).

Japan Taiwan Korea United States Canada

Pole and Lon~- Gill Other Long- Gill Long- Gill Pole and Gill Grand

Year linea line netC gear Total line netd Total linee netd Total line Troll! Sport net Total Troll Total

1952 41,786 26,687 237 68,710 23,843 1,373 25,216 71 93,997

1953 32,921 27,777 132 60,830 15,740 171 15,911 5 76,746

1954 28,069 20,958 38 49,065 12,246 147 12,393 61,458

1955 24,236 16,277 136 40,649 13,264 577 13,841 54,490

1956 42,810 14,341 57 57,208 18,751 482 19,233 17 76,458

1957 49,500 21,053 151 70,704 21,165 304 21,469 8 92,181

1958 22,175 18,432 124 40,731 14,855 48 14,903 74 55,708

1959 14,252 15,802 67 30,121 20,990 0 20,990 212 51,323

1960 25,156 17,369 76 42,601 20,100 557 20,657 5 63,263

1961 18,636 17,437 268 36,341 2,837 12,061 1,355 16,253 4 52,598

1962 8,729 15,764 191 24,684 1,085 19,760 1,681 22,526 1 47,211

1963 26,420 13,464 218 40,102 2,432 25,147 1,161 28,740 5 68,847

1964 23,858 15,458 319 39,635 26 26 3,411 18,392 824 22,627 3 62,291 b;'

1965 41,491 13,701 121 55,313 16 16 417 16,545 731 17,693 15 73,037

1966 22,830 25,050 585 48,465 16 16 1,600 15,342 588 17,530 44 66,055

1967 30,481 28,869 520 59,870 17 17 4,113 17,826 707 22,646 161 82,694 Q..

1968 16,597 23,961 1,109 41,667 15 15 4,906 20,444 951 26,301 1,028 69,011 i 1969 32,107 18,006 1,480 51,593 21 21 2,996 18,839 358 22,193 1,365 75,172

1970 24,376 15,372 956 40,704 23 23 4,416 21,041 822 26,279 354 67,360

1971 53,198 11,035 1,262 65,495 24 24 2,On 20,537 1,175 23,783 1,587 90,889 Z

1972 60,762 12,649 1 921 74,333 25 25 3,750 23,608 637 27,995 3,558 105,911 0

... 1973 69,811 16,059 39 1,883 87,792 35 35 2,236 15,667 84 17,987 1,270 107,084 ;.

1974 73,576 13,053 224 1,065 87,918 40 40 4,777 20,187 94 25,058 1,207 114,223 '"=

1975 52,157 10,060 166 402 62,785 28 28 319 319 3,243 18,975 640 22,858 101 86,091 (")

1976 85,336 15,896 1,070 1,394103,696 37 37 971 971 2,700 15,932 713 19,345 252 124,301 5(")

1977 31,934 15,737 688 1,039 49,398 61 61 65 65 1,497 10,005 537 12,039 53 61,616

1978 59,877 13,061 4,029 3,209 80,176 53 53 174 174 950 16,682 810 18,442 23 98,868 0-

1979 44,662 14,249 2,856 1,280 63,047 81 81 27 27 303 6,801 74 7,178 521 70,854 (")

1980 46,743 14,743 2,986 1,516 65,988 15 15 382 7,574 168 8,124 212 74,339 0

1981 27,426 18,020 10,348 959 56,753 600 600 748 12,694 195 13,637 200 71,190 t'l

1982 29,615 16,762 12,511 1,054 59,942 1,070 1,070 425 6,661 257 7,343 104 68,459 (")

0 1983 21,098 15,103 6,884 471 43,556 1,233 1,233 607 9,512 87 10,206 225 55,220 0'

1984 26,015 15,III 10,569 3,898 55,593 2,708 2,708 1,030 9,378 1,427 15,563g 50 73,914

1985 20,714 14,320 13,132 1,940 50,106 5,447 5,447 1,498 6,431 1,176 2 9,109 56 64,718

1986 16,096 12,945 9,749 2,192 40,982 432 4,708 196 3 5,339 30 46,351 Q..

1987h 19,091 14,642 7,617 1,394 42,744 158 2,766 74 5 3,003 104 45,851 0

1988h 7,000 12,000 11,000 11,000 598 4,212 64 15 4,889 85 27,974 (")

."

0

a Japanese pole-and-line catches include fish caught by research vessels.

bJapanese longline catches for 1952-60 exclude minor amounts taken by vessels under 20 tons. Japanese longline catches from 1958-68 were readjusted in ]988. Longline

catches in weight are estimated by multiplying annual number of fish caught by average weight statistics.

C Japan gillnet catches include south Pacific catches.

d Taiwanese and Korean gillnet catches are missing or incomplete.

e Korean longline catches calculated from FAO statistics and Korean catch/effort data. I -..J

! U.S. troll catches from 1952-60 include fish caught by baitboats, from 1961-85 include fish landed in Hawaii. U.S. jig (troll) catches (1984-88) include gillnet catches. \,>0

g U.S. total for 1984 includes 3,728 mt caught by purse seines.

h Figures for 1987-88 are preliminary.
74 NOAA Technical Report #105

.'

120'

60'

50'

40' ...

30'

20'

140'

A

140'

160'

.,

54% ......---

WEST OF 1800

160'

180'

180'

'..t>

160'

OTHERS: 5%

140' 120'

20'

10'

180'

.." :,.....

40'

10%

30' WEST OF 1800

.'

20' '.. 20' t>

OTHERS: 4%

10' 10'

B

O'

140' 160' 180' 160' 140' 120'

Figure 4

(A) Percent of total recoveries of tagged albacore for fish released in coastal waters off North America north of 40° N.

Release area is indicated by hatching. (B) Percent of total recoveries of tagged albacore for fish released in coastal

waters off North America south of 40° N. Release area is indicated by hatching.
Figure 5

(A) Positions of release (dots) and recovery (arrowheads) of tagged albacore which were recovered within the season of release during 1972. (B) The same as A

except for recoveries in the subsequen t fishing season, 1973.

i

c.

j

Z

o.,

er

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o

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o

4~:5° 130· 125· 120· 115· 140· 135· 130· 125· 120· • I I j I I I I I i lal lAm.. iii i i ,48·1 1 1 I. 115 48• I I I I I I, 11\1 _I i I I I 1 I

I

I

"'STOR'A j[ I i '",-, ! I;ASTOR,"

45. ALBACORE TAGGED a • I / .'. mRELEASED IN JULY _ 45 I / i'ALBACORE TAGGED a 45·

?AUGUST 1972, NORTH OF / I ,RELEASED IN JULY-AUG

:. 32. N LATITUDE AND / i : 1972 NORTH OF 32° N. a , RECOVERED DUR'NG ~72. / lRECDVERED DURING '973 ~c. BLANCO , I

J

I

\

I I

RECOVERED OFF JAPAN ----..:. \

40. \.\. '\ • \ \~..c ".E"OOCINO -l 40 I- I \' 40·

--. \ \ I "

\, \ I I ':. , I I \ '

, \ I I , \ I \ ',\ \ \ ,\ I \ ,\ I

'\ 1\ ',\ \I

" II

35· \ m".~:~t. ._ ,..__.... --I 35 I- "II ~~lI...... ",.... -' 35· ,M --.

I'~r 'tv I .•:\'t,~, I

i~'~\

.. \ \ '!'SAN O'EGO

30"1 ~30'1 \1- .

lOl, ","" """'" ~lOJ , , ,I '" I " ,I"" I , , , ,~ , 135 130 125· 1200 115· 140· 1350 1300 1250 120· 115}5

-.J

\1t
76 NOAA Technical Report #105

III the southern area, 78 percent were recovered in

the same general area as release, 10 percent west of

180°, 8 percent in coastal waters off North America

north of 40° N, and about 4 percent in the central

eastern Pacific or unknown area of recovery (Figure

4B).

The tagging results suggest that the northern subgroup of albacore is primarily exploited by the

Japanese surface fishery in the western Pacific, the

Asian longline fishery and the portion of the North

American fishery operating north of about 40° N.

The southern subgroup appears to be harvested

mainly by the North American surface fishery operating south of about 40° N and the Asian longline

fishery, and only limitedly by the Japanese pole-andline fishery.

Release and recovery locations for tagged albacore

that were recovered by U.S. fishermen within the

same fishing season as release and the fishing season

following release are shown in Figure 5, A and B,

respectively. These results reveal that the movement

patterns of albacore are highly directed, that the fish

follow well-defined routes during intra- and inter-fishery migrations. Furth~r evidence for this is indicated

by the relatively high proportion of tagged fish that

have been recovered within 150 miles of where they

were released off the coast of North America after

being at liberty for one to three fishing seasons

(Laurs and Nishimoto 1974). Presumably these fish

had migrated seasonally between coastal waters off

North America and the central or western Pacific.

For example, one tagged fish was recovered only 39

miles from where it had been tagged nearly two years

earlier. Tagged albacore recoveries made by the Japanese pole-and-line fishery also show a pattern

indicative of a directed, well-defined migration route

(Laurs and Nishimoto 1974).

Physiological Ecology and

Habitat Definition _

Most of the present understanding of albacore habitat has been gained through studies at sea following

multi-disciplinary approaches including physical and

biological oceanography, satellite oceanography,

acoustic tracking, and physiological ecology research.

The latter investigations have shown that the albacore is a highly advanced teleost with many

specialized adaptations. It is a thermo-regulati:1g endotherm (Graham and Dickson 1981), has a high

metabolic rate (Graham and Laurs 1982), an advanced cardio-vascular system (Breich et al. 1983; Lai

et al. 1987), specializations in the circulatory system

and blood/gas exchange system (Laurs et al. 1978;

Alexander et al. 1980; Cech et al. 1984; Graham et al.

1989), distinctive enzyme and complement systems

(Morrison et al. 1978; Giclas et al. 1981; Dyke et al.

1987; Dickson 1988), and high energetic costs for migration (Sharp and Dotson 1977), which may be

partly met by utilization of stored fat (Dotson 1978).

Recent research refutes earlier published information on temperature preference, which was based on

the belief that albacore were confined to the upper

mixed layer and that the thermocline formed a barrier to their vertical distribution (Clemens 1961).

Based on these assumptions, modal sea surface temperatures (SST) measured where highest troll

catches were made, generally between 16° and 19° C

(e.g., Clemens 1961), were thought to represent the

temperature preference for pre-adult albacore in the

eastern Pacific. However, acoustic tracking of freeswimming albacore and concurrent measurement of

ocean vertical thermal structure (Laurs et al. 1980;

Laurs and Dotson, in prep.) have demonstrated that

3 to 5 year-old albacore spend most of their time

swimming in or near the thermocline. The fish spend

small amounts of time in the upper mixed layer, presumably only when enticed there to feed. The

tracked fish moved through waters with a temperature range of about 10° to 19° C and spent most of

the time in waters with temperatures much cooler than

those originally thought to be their optimal range.

The tracked fish also exhibited marked vertical excursions in depth; the range generally was larger

during the day than at night when in California Current waters (Fig. 6). However, two fish tracked in

offsore waters, which are believed to have been actively migrating, showed larger depth excursions

during the night, which often brought them up into

the lower portion of the mixed layer. When making

vertical excursions, albacore pass through large temperature gradients, routinely 4° to 6° C and up to

10° C, within about twenty-minute periods.

Based on the acoustic tracking results that albacore

seldom enter into waters cooler than 10° C and the

finding of Graham and Dickson (1981) that thermoregulation processes begin to fail at temperatures

below 10° C, it appears that this temperature is the

lower limit of the preferred temperature range. At

this point of our understanding, we believe that the

normal habitat of albacore is within a temperature

range of about 10° to 20° C, in waters with a dissolved

oxygen saturation greater than 60 percent (Graham

and Laurs 1982). However, the SST range of 16° to

19° C still has value in fishing operations.

There is ample evidence that the migration, distribution, availability, and vulnerability of albacore are

markedly influenced by oceanographic conditions in

the North Pacific Ocean, notably fronts. Albacore
Albacore 79-3 July 31-August 1, 1979

Figure 6

A time plot of the vertical movements of an acoustically tracked albacore in California Current waters. Vertical temperature field is based upon concurrent XBT casts.

[

§

0-

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c 200

250

50

150

100

o

300

340

10'

18.6'

9'

2355

I

MOORset

- 8'

18.8'

Sunset

18.8'

Hoole-up on a

Jig Fish on Stern

7'

18.7'

-

'1200

TIME (PDT) Moonrise

2/3 moon

Catch a Fish on

Surface R & R

8' _

18.6'

Sunrise

7'

18.8'

0400

18.8'

0000

Moonset

Porpoise School

Nearby

18.6'

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o 18' 19':= C 19' I-

-' Ii • 16 __ - 16'

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10' - __ - -, 1~--~t1T11 1\1 III - ~~,

-10' -U\-Ul \I

50

100

150

200

300

250

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78 NOAA Technical Report #105

fishing grounds in the western Pacific have been

linked to oceanic fronts (Uda 1973). Also, the seasonal migration of albacore into North American

coastal waters has been found to be associated with

the North Pacific transition zone waters and its frontal zone boundaries (Laurs and Lynn 1977).

Oceanographic conditions also play important roles

in the local concentrations and movements of albacore in coastal waters off North America. Albacore

tend to aggregate on the warm side of upwelling

fronts and to move away from the locations where the

fronts occurred when upwelling breaks down (Laurs

et al. 1977). Satellite images of ocean color and SST

and concurrent albacore catch data also clearly show

that the distribution and availability of albacore in

nearshore waters off California are related to coastal

upwelling fronts. Albacore are most abundant in

warm, clear, blue oceanic waters adjacent to the temperature and color fronts which form the seaward

boundary of the relatively cool, turbid coastal water

masses (Laurs et al. 1984). Laurs et al. (1984) found

that, in waters beyond about 500 miles from the

coast, fishing success was related to the productivity

of the waters. Ocean boundaries associated with the

Columbia River plume also appear to be important in

the aggregation of albacore (Pearcy and Mueller

1970) .

It is presumed that albacore aggregate in the vicinity of upwelling fronts to feed on forage organisms

that are plentiful in these areas (Blackburn 1969;

Laurs et al. 1977). Yet, it remains unclear what physical factors prevent albacore from penetrating these

fronts in order to reach what would likely be the

highest potential forage biomass (Blackburn 1969).

Because tuna can perceive temperature gradients as

small as 0.1° C (Steffel et al. 1976), thermal-physiological mechanisms have been thought to be the

main limitation to tunas crossing sea surface· temperature gradients into cooler waters. Neill (1976)

postulated .that tunas utilize frontal gradients for behavioral thermoregulation. Past studies have also

stressed confinement to physiological optimal temperature range (Thompson 1917; Clemens 1961;

Sund et al. 1981). However, the findings that albatore can regulate their body temperatures over a

relatively broad range and that they may routinely

pass through vertical temperature gradients of up to

10° C, clearly substantiate that the aggregation of albacore on the warm, clear side of SST fronts is not

related to thermal-physiological mechanisms.

Recent research involving acoustic teleme:ry of

free-swimming albacore, satellite measurements, and

oceanographic sampling indicate that water clarity as

it affects the ability of albacore to detect prey may be

the causal mechanism underlying the aggregation of

albacore on the warm, clear sides of upwelling fronts

(Laurs, in prep.). In offshore regions, the distribution of albacore in relatively productive waters of the

transition zone is believed to occur because relatively

higher amounts of food organisms are present than

in the Central Pacific gyre waters, yet the waters are

clear enough for the albacore to see their prey.

Food Habits of Albacore in

Transition Zone Waters _

Information is very limited about the food habits of

albacore in the central North Pacific. However, they

appear to be opportunistic carnivores, as they are in

other areas (e.g., McHugh 1952).

Iverson (1962) reported on the stomach con tents

of 79 albacore caught by trolling and 87 caught by

gill net in the area bounded by longitudes 140° W

and 180° and latitudes 32° and 47° N. The fish

ranged in fork length from 51 to 85 cm and were

caught on research cruises conducted between 1950

and 1957. The average displacement volume of food

per stomach in troll-caught fish was 15.1 mL and in

gillnet-caught fish was 9.8 mL. The author speculated

that there was less food in the stomachs of the gillnetcaught fish because the fishing was done at night and

albacore are not known to feed at night, and because

the fish may have vomited excessively while struggling in the net. Laurs and Nishimoto (1973) and

Nishimoto and Laurs (1974) reported on the stomach contents of 33 and 75 albacore caught in the

transition zone area between 130° and 140° W during

May of 1973 and 1974, respectively. Stomachs were

examined in both years from fish ranging in size

from 50 to 85 cm FL. Food was found in all stomachs

and averaged 16.3 mL displacement volume for fish

examined in 1973, and was found in 88 percent of

the stomachs and averaged 13.3 mL for fish examined in 1974.

Variations are apparent in the frequency of occurrence and percent volume of the major food groups

found in the albacore stomachs in the studies reported above (Table 2). Cephalopods had the

greatest biomass in two of the analyses and saury the

greatest in the other two. For all the analyses combined, cephalopods occurred in nearly 57 percent of

the stomachs examined and comprised approximately 38 percent of the food by volume. Fishes

(other than saury) and crustaceans were found in

nearly the same number of stomachs, about 48 and

42 percent, respectively, but contributed only about

14 and 11 percent of the volume of food, respectively.
Laurs and Lynn: North Pacific Albacore Ecology and Oceanography 79

Table 2

Summary of albacore feeding habits in transition zone waters. (Displacement volumes in mL.)

Gillnet Gillnet Gillnet Gillnet All studies

caughta caughtb caughtC caughtd combined

Food Group

% occur/vol. % occur/vol. % occur/vol. % occur/vol. % occur/vol.

Saury 11.5 26.5 25.3 61.5 10 38.2 10 0.3 14.2 31.6

Other fish 10.3 8.0 31.6 16.9 80 18.0 65 14.8 46.7 14.4

Cephalopods 28.7 62.1 53.1 11.3 70 20.1 75 59.1 56.7 38.2

Crustacea 5.7 1.2 41.7 6.5 60 16.7 60 19.5 41.8 11.0

Others and

Unidentifed 2.1 3.8 35 7.0 25 6.3 4.8

a Iverson (1962) n =87; mean volume =9.8 mL.

b Iverson (1962) n = 79; mean volume = 15.1 mL.

C Laurs and Nishimoto (1973) n = 33; mean volume = 16.3 mL.

d Nishimoto and Laurs (1974) n = 75; mean volume = 13.3 mL.

Because of their large size, saury, which occurred in

only about 14 percent of the stomachs examined,

composed nearly 32 percent of the biomass of food

found in the stomachs.

The number of albacore stomachs that were examined in the investigations discussed above was

relatively small, and care must be exercised in drawing conclusions concerning the feeding habits of

albacore in the transition zone waters. However,

some trends are evident and it is possible to make

tentative comparisons with the results of investigations of feeding habits of albacore caught in waters

closer to shore in the California Current (McHugh,

1952; Pinkas et al. 1971; Bernard et al. 1985). Larval

fishes from several families, juvenile lanternfishes,

and often times small-eye squaretail (Tetragonurus

cuvieri), carangid fishes, and amphipods are more important in the diet of albacore in the offshore

transition zone waters than in those caught closer to

shore in the California Current. In the latter region,

saury or anchovy (or both) and euphausiids or

sergestid shrimps are generally more important.

Squids are important in both regions, but the species

composition is different. While the composition of

the food in the stomachs is different between inshore

and offshore, the average volume of food in stomachs

from the two regions is similar. For example, in an

examination of 262 stomachs of albacore caught by

trolling in waters along the Pacific coast of the

United States within a few hundred miles of shore,

the volume of food averaged 15.0 mL displacement

volume per stomach (Laurs and Nishimoto, in

prep.). This compares with displacement volumes

ranging between 13.3 mL and 16.3 mL for albacore

caugh t by trolling in the Transition Zone (Table 2).

North Pacific Albacore in Relation

to Transition Zone Oceanography _

North Pacific transition zone waters lie between the

cool, low salinity Pacific Subarctic waters to the north

and the warm, saline North Pacific Central waters to

the south and have temperatures and salinities characteristic of a mixture of these two water masses

(Sverdrup et al. 1942). Transition zone waters are

found in a band across the North Pacific middle latitudes within the North Pacific Current and are

bounded by sharp discontinuities in temperature and

salinity at the surface, extending to depths of about

the halocline (McGary and Stroup 1956; Roden 1975;

and Lynn 1986). These bounding gradient regions

are often times referred to as the subarctic front at

the north and the subtropical front at the south

(Fig. 1), and each may comprise a complex series of

fronts. The dynamic processes which produce and

maintain these gradients also enrich these waters

(McGary and Stroup 1956).

The association of albacore with transition zone

waters in the central North Pacific was initially suggested by Shomura and Otsu (1956), Graham (1957)

and McGary et al. (1961). These authors reported on

the results of a two-year program, 1954 and 1955, in

which a series of eight exploratory fishing surveys in

various seasons and three oceanographic surveys

were conducted in the central temperate North Pacific. The fishing methods used were longlining,

trolling, and a brief experiment with gill netting. Although catches were small, there was a clear

association with the oceanographic regime of the

transition zone and a significant latitudinal shift with

season.
80 NOAA Technical Report #105

40"

45·

CATCH/ISO LINE HOURS

x No Calch

o 1-5

• 6-50

• 51-100 CAPE • 101-200 MENDDaND

e 201-300

• >301

o ...

x

x

00

• x

.~

30· 30·

A

25· 25·

ISO· 115° 48· 48·

35·

x

.

x x

40·

45·

Figure 7

(A)Jig-boat catches of albacore and oceanic fronts for

June 1973 (from Laurs and

Lynn 1977). (B) Jig-boat

catches of albacore and oceanic fronts for June 1974

(from Laurs and Lynn

1977).

45· 45·

CATCH/I50 LINE HOURS

x No Catch

0 1-5

· 6-50

• 51-100

CAPE • 101-200 MEHDOCWO

40· • 201-300 40·

• >301

35·
Laurs and Lynn: North Pacific Albacore Ecology and Oceanography 81

STATION 34 36 38 40 42 44 46 48 50

O-r-----'mF.':':'~ml:w---'--...J....-'----___'_r-....,.~~".."I_I7'""'?+:O_,?"J.,_,,....,.L'7'""'7..,.,....~J,_,._T_~+.,....,.'--------.,._

- 100

Cl>

Q;

E

I

Q.

W

o

200 )

34.1

40"N

STATION 34 36 38 40 42 44 46 48 50 0-r---......L..-'----.....J;r-..L...-~--'----l-......L..-"r_--'--~~-......,.----'-....,....L...-~--L...----.,_

100

2!

Cl>

E

I

0..

W

0

200

/

8

Figure 8

Vertical sections of temperature and salinity along 137°30' W forJune 1973 (from Lynn 1986).

Based on extensive oceanographic sampling and

concurrent albacore fishery data in the eastern

North Pacific, Laurs and Lynn (1977) demonstrated

that the distribution, relative abundance, and seasonal migration of albacore in the eastern Pacific are

related to the transition zone waters. In a series of

late spring surveys conducted in 1972 through 1976,

albacore were caught by chartered commercial fishing boats, while a research vessel made detailed

oceanographic measurements. The summaries of re-
82 NOAA Technical Report #105

60'

50'

40'

120' 140'

50'

40'

20' ..... ....

'..c; .. . . .... 20'

SALINITY at 90m

10'

..... ..... O'

160' 180' 160' 140' 120'

Figure 9

Selected isolines of the salinity distribution at 300 feet (90 meters) (from Robinson 1976).

suits for June 1973 and June 1974 are given in Figure

7, A and B. The shaded bands indicate oceanic fronts

in plan view, at which there were strong horizontal

gradients of temperature and salinity. The solid

circles show albacore daily catch rates, using a scale

given in the legend. There was a well-defined relationship between high albacore catch rates and

fronts. In 1973 the fronts were strong and catches

were confined between them. In 1974 the fronts were

weak and diffuse, and catches Were spread over a

greater region and showed considerable movement

over short periods during the month. Despite the

weaker development of the ocean fronts, there was

still a clear association of albacore with the fronts.

Vertical sections of salinity and temperature along

a meridian for 1973 show the water masses and the

fronts that define their boundaries (Fig. 8). The definition is clearest in the salinity structure. Water

having its source in the subarctic is low in salinity

(hatched) and that from the subtropics is high in

salinity (shaded). Although there is a great deal of

variability, these fronts are semi-permanent and

quasi-continuous across the North Pacific. Specific

isohalines are traditionally associated with each front

(e.g., Roden 1980; Lynn 1986). The average salinity:

contours at 90 m (Robinson 1976) for the North Pacific are roughly zonal (Fig. 9) and show some

curvature similar to the large-scale pattern of the

North Pacific current field. The 33.8 isohaline is

found in the subarctic front. The 34.6 isohaline is

located in the Kuroshio Extension front and in the

northern subtropical front in the eastern North Pacific. The 34.8 isohaline is situated in the subtropical

front. The meanders in the 34.6 isohaline coincide

with major bathymetric features, the Shatsky Rise, the

Emperor Seamounts, and the Hess Rise (Fig. 10),

showing the strong influence that these major features have upon surface currents and distributions

(see Roden and Taft 1985). The schematic of the

frontal system in the temperate zone (Fig. 1) was

drawn using this smoothed version of salinity and a

large number of specific observations of fronts from

research cruises (most conducted by the International North Pacific Fisheries Commission, the

University of Washington [G. Roden] and the Southwest Fisheries Center). This is not unlike a similar

schematic developed by Roden (1975). It does, however, add the northern subtropical front first

iden tified by Lynn (1986) . For a major portion of the

year, these fronts apparently are an important mechanism for increasing biological productivity in the

central portions of the North Pacific Ocean, which

lead to higher levels of forage organisms for albacore.

At any particular time and place, the fronts associated with the transition zone may have large
Laurs and Lynn: North Pacific Albacore Ecology and Oceanography 83

Figure 10

Bathymetric features ofthe North Pacific as given by the 2000 fathom isobath.

. . .~~.. ,... ...--c , ... ... N

20 YEAR AVERAGE

16<18°C SEA SURFACE TEMPERATURE

...., ' : ,·-l10·

60'

50'

40'

30'

120'

.... · ·:·.. · · ·; ;·~O·

160' 180' 160' 140'

120'

10'

Figure 11

Distribution ofthe 20-year average sea surface bands of temperature between 16 and 18° C for February, May, and August.
1,600

00

Z 0 I

/')

400 [ l;; a

:;l:l :I:

,800 li: "0

0..,

....

:II:

1,200 0-\>1

12S"W

VOYAGE 43

ISS"W ISO"W 14S"W 140"W 13S"W 1300W

MAY 27-31,1972

~HONOLULU NAUnCAL MILES SAN FRANCISCO,

2,000 1,500 1,000 500 0 , I I iii

400

SOO 1,600

12S"W

VOYAGE 40

13S"W 13COW

.• I I ,0

ISO"W 14S"W 140"W

/7~

ISS"W

~7_

~HONOLULU NAUTICAL MILES SAN FRANCISCO,

2,000 1,500 1,000 500 0,

APRIL IS -19, 1972

0, < i , I

4001

SOO

100 100

400

iii >: iii a: a:

'" \oJ \oJ

:;; 200 \oJ

u. :;; 200

! - ! :I:

:I: 800 t- :I:

t- a. t- a. '" a.

~3oo 0 ~3oo

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LONGITUDE LONGITUDE

JUNE 24 -28,1972 VOYAGE 4S JULY 22-26,1972 VOYAGE 47

-HONOLULU NAUTICAL MILES SAN FRANCISCO2,000 I,SOO 1,000 SOO 0 , , Iii i

~HONOLULU NAUTICAL MI LES SAN FRANCISCO...,

2,000 I,SOO 1,000 SOO 0 i I I Iii

0 0 0 0

100 100

400 400

Vi ;: iii a: a: t-

'" '" '" '" t;; 200 '" t;; 200 '" ::E ::E - :I: - :I: :I: 800 t- :I: 800 li: t- a. t- a. '" a. '" '" 0 300 <> <> 300

~200 ~200

4001-

'" - /~ / ./" , 400-

S~ 7,600 soo ~600

I I I I I i I ISS"W ISCOW 14S"W 140"W 13S"W 130"W 12S"W ISS"W ISO"W 14S"W 140"W 13S"W 13COW 125"W

LONGITUDE LONGITUDE

Figure 12

Vertical sections of temperature for a great circle path between Honolulu and San Francisco for four periods between April and July 1972.
Laurs and Lynn: North Pacific Albacore Ecology and Oceanography 85

meanders, be split into multiple fronts; or be dissipated. The schematic in Figure I is meant to describe

a base pattern that may not be evident at the surface

at all times, but likely can be found in the upper 100

or so meters. The surface mixed layer undergoes

strong seasonal changes in temperature. In particular, during late summer and fall the surface may not

reflect the deeper frontal structure. During winter,

cooling and deep mixing reestablish the fronts in the

surface layer.

Peak catches of albacore in the U.S. surface fisheries generally occur where SSTs are between 16° and

19° C (Clemens 1961). Plots of surface temperatures

within this latter temperature range for February,

May, and August show the seasonal progression of

warming (Fig. 11). In some seasons and some regions

this temperature range coincides with the oceanic

fronts; in other seasons and regions they are greatly

separated. For instance, in the western Pacific in May,

the temperature range coincides with the Kuroshio

Extension front and, in the eastern Pacific during

summer, it falls well to the north and east of the

fronts associated with the transition zone.

The seasonal warming of the surface layer and the

development of the summer thermocline is shown in

a series of panels of temperature versus depth along

a great circle route between Honolulu and San Francisco for April through July, 1972 (Fig. 12);

temperatures between 16° and 19° C are shaded. In

April there is a deep mixed layer and winter thermocline. The surface warming indicated in the

subsequent panels, forms a shallow surface layer in

which the waters characteristic of albacore surface

fisheries advance toward the California coast. This

development is coincident with the local development of the U. S. troll fishery.

Catches and Ocean Features _

The center of the largest catches by the Japanese

longline fishery based on 5° square data summaries

falls in a band of latitude between 25° and 35° N, and

extends from Japan to the dateline (Fig. 2A). A second band is found north of Hawaii between 30° and

35° N. The largest catches are coincident with the

estimated mean position of the subtropical front, 30°

to 31°N (Fig. 1). Winter surface temperatures at

these locations are higher than 15° C; hence, a layer

of water that has temperatures that include the upper

end of the "favored" range lies above the deep winter

themocline (Fig. 12). The zonal division in longline

catches in the area located at 1700 W to 180°, appears

well documented in the data records. It appears to be

related to a dowpstream effect of major bathymetric

features (Fig. 10) upon the current field and oceanic

structure. This division in catches may offer an explanation for the development of subgroups of North

Pacific albacore. If large-scale oceanic structure divides the wintering habitat, separate migration paths

and related consequences may be the result.

The center of the largest catches of the Japanese

pole and line fishery progresses from near the coast

of Japan during April to 175° E by June (Fig. 2A).

The large catch nucleus is found within the Kurshio

extension front (Fig. 1) and follows the northward

advance of warming, which forms a shallow surface

layer over the transition zone waters north of the

Kuroshio front (Fig. 11). The core of high catches

near 170° E is probably related to events controlled

by the bathymetry of the rises and seamounts.

The U.S. jig surface fishery starts in May near the

Hess Rise, centered at about 180° and 35° N (Fig 2A).

The appropriate surface temperatures coincide with

the northern subtropical front, or eastward continuation of the Kuroshio extension front. Subsequently,

the surface warming and the fishery shift northward

as the migration of albacore continues eastward. By

July the seasonal warming creates favorable temperatures north and east of the subarctic front. A fishery

subsequently develops offshore of coastal upwelling

regions off the coast of North America.

Citations

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Bernard, H.,j.B. Hedgepeth, and S.B. Reilly.

1985. Stomach contents of albacore, skipjack, and bonito

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1943. Contribution to the biology of the albacore (Germo

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1988. Why are some fishes endothermic? Interspecific comparisons of the aerobic and anaerobic metabolic capacities

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1978. Fat deposition and utilization in albacore. In The

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1987. Isolation and characterization of a new zinc-binding

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Ulevitch.

1981. The complement system of the albacore tuna, Thunnus

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1981. Physiological thermoregulation in the albacore tuna

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1982. Metabolic rate of the albacore tuna Thunnus

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1989. O2 tension, swimming-velocity, and thermal effects on

the metabolic rate of the Pacific albacore TI.unnus

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Graham,J.J.

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1989. Mitochondrial DNA

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Iversen, R.T.B.

1962. Food of albacore tuna, Thunnus gerrno (Lacepede), in

the central and northeastern Pacific. Fish. Bull., U.S.

214:459-481.

Lai, N.C.,].B. Graham, W.R. Lowell, and R.M. Laurs.

1987. Pericardial and vascular pressures and blood flow in

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192.

Laurs, R.M.

1983. The orth Pacific albacore-an important visitor to

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Laurs, R.M.

In prep. Albacore catches in relation to water clarity and temperature fronts off California. NMFS Southwest Fish Sci.

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Laurs, R.M., and R. Nishimoto.

1973. Food-habits of albacore caught in offshore area. In Report of Joint National Marine Fisheries Service-American

Fisherman's Research Foundation albacore studies conducted during 1973, p. 36-40. NMFS Southwest Fish. Sci.

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Laurs, R.M., and R.N. Nishimoto.

1974. Joint NMF5-AFRF albacore tagging study. In Report of

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Laurs, R.M., and R.J. Lynn.

1977. Seasonal migration of North Pacific albacore, Thunnus

alalunga, into North American coastal waters: Distribution,

relative abundance, and association with Transition Zone

waters. Fish Bull., U.S. 75:795-822.

Laurs, R.M., and R. Nishimoto.

1979. Summary of findings made by vessels on charter to the

Pacific Tuna Development Foundation. MFS Southwest

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Laurs, R.M., and].A. Wetherall.

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302.

Laurs, R.M., and R. Dotson.

1983. Exploratory longline fishing for albacore tuna ineastern North Pacific waters during November-December

1982. NMFS Southwest Fish. Sci. Center Adm. Rep. LJ-83-

5,71 p.

Laurs, R.M., and R. Dotson.

In prep. The vertical distribution and movements of freeswimming albacore tuna, Thunnus alalunga (Bonnaterre)

measured by ultrasonic telemetry, in relation to vertical

ocean thermal structure in the eastern North Pacific. NMFS

Southwest Fish. Sci. Center.

Laurs, R.M., and R. Nishimoto.

In prep. Stomach contents of troll-caught albacore in comparison to micronekton from net tows in the eastern North

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Laurs, R.M., H.B. Clemens, and L.H. Hreha.

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1961-1970. Mar. Fish. Rev. 38:1-32.

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in relation to ocean features as indicated by ultrasonic

tracking and oceanographic sampling. Fish. Bull., U.S.

75:347-355.

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1978. Estimates of blood volume in albacore tuna. In The

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Laurs, R.M., P.C. Fiedler, and D.R. Montgomery.

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Morrison, D.C., RM. Laurs, and RJ. Ulevitch.

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1963. Model of the migration of albacore in the North' Pacific Ocean. Fish. Bul!., U.S. 63:33-44.

Parrish, R.H., N.W. Bartoo, S.R. Herrick Jr., P.M. Kleiber, R.M.

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NOAA-TM-NMFS-SWFC-126, 56 p.

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1970. Upwelling, Columbia River plume and albacore

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Univ. Michigan, Ann Arbor, p. 1I0l-l113.

Pinkas, L., M.S. Oliphant, and I.L.K. Iverson.

1971. Food habits of albacore, bluefin tuna, and bonito in

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1976. Atlas of North Pacific Ocean and mean salinities of the

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NOO RP-2, i-xix, 173 figs.

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1975. On North Pacific temperature, salinity, sound velocity

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1980. On the subtropical frontal zone north of Hawaii during the winter. J. Phys. Oceanogr. 10:342-362.

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1985. Effect of the Emperor Seamounts on the mesoscale

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1977. Energy for migration in albacore, Thunnus

alalunga. Fish. Bull., U.S. 75:447-450.

Shomura, R.S., and T. Otsu.

1956. Central North Pacific albacore surveys, January 1954-

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173, 29 p.

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Sverdrup, H.U., M.W.Johnson, and R.H. Fleming.

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1917. Temperature and the albacore. Calif. Fish Game

3:153-159.

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1973. Pulsative fluctuation of oceanic fronts in aSSOCiatIOn

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1987. Growth variation and stock structure in North Pacific

albacore. Working paper NPALB/87/21, tenth North Pacific albacore workshop, Shimizu,Japan, 21 p.

Zonal Variability in Salmonid Driftnet Catch Rates

in the Central North Pacific Ocean

STEVE IGNELL

Auke Bay Laboratory

Alaska Fisheries Science Center

National Marine Fisheries Service

National Oceanic and Atmospheric Administration

11305 Glacier Highway, juneau, Alaska 99801

ABSTRACT

Data from 17 Canadian, Japanese, Taiwanese, and Republic of Korea research vessel

cruises conducted between 1983 and 1987 were examined for salmonid catches along the

northernmost one-half degree latitude of the Japanese squid-driftnet fishing area. A total

of 325 salmon (Oncorhynchus spp.) and 2 steelhead trout (0. mykiss) were caught in 11 of

30 driftnet operations, resulting in an overall catch of 0.127 salmonids per 50 m (tan) of

driftnet. Chum (0. keta) and coho (0. kisutch) salmon composed 60% and 34%, respectively, of the total salmonid catch. The proportion of sets west of 1750 W encountering

salmon was 53%, compared to 9% east of 1750 W longitude. Catch per tan in the western

and eastern regions was 0.168 and 0.006, respectively, a 28-fold difference between regions. When the data were stratified further by sea surface temperature interval, the

zonal cline in salmonid catch rates persisted.

Introduction _

In the early 1980s, the Government of Japan established regulatory measures for their flying squid

(Ommastrephes bartrami) driftnet fishery in the central

North Pacific Ocean. These measures provided a

fishing season extending from June through December and restricted vessel operations between 145 0

W

and 1700 E longitude, north of 20° N latitude and,

depending upon the month, south of 40°, 42°, 44°,

or 46° N latitude (Fisheries Agency of Japan 1982).

The monthly moving northern boundary was set to

approximately coincide with the mean monthly position of the 15° C sea surface temperature (SST)

isotherm. Such a provision was believed to restrict

fishing to areas where salmonid abundance

(Oncorhynchus spp.) is low, thereby minimizing the

impact of this fishery on salmonid stocks. (Takagi

1983) .

Several studies have been conducted in recent

years to assess the effectiveness of these regulations

in minimizing impacts to salmonids. Of primary interest has been the relationship of salmonid catch

per unit effort (CPUE) with respect to SST and the

distributional overlap of flying squid and salmonids

(Takagi 1983; Dahlberg and Sigler 1987; Harris and

Kautsky 1987; and Ogura and Takagi 1987). In general, these studies have shown some overlap in

salmonid and flying squid distribution, negligible salmonid CPUE in SSTs above 14-15° C, low squid

CPUE in SSTs below 12° C, and an encounter rate of

flying squid north of the northern boundary of the

squid driftnet fishery that is higher than the encounter rate of salmonids south of the boundary. The

studies have also included analyses of data from a

small number of driftnet operations that have shown

salmonid harvest within the squid area to be infrequent and low in magnitude, especially during years

when SSTs are warmer than average. Despite the paucity of oservations from the regulatory area, most

authors have concluded that the northern boundary

is an effective management measure for limiting incidental catches of salmonids (Takagi 1983; Harris and

Kautsky 1987; and Ogura and Takagi 1987).

Other studies have analyzed how the oceanographic features characteristic of the northern

boundary of the squid regulatory area are potentially

associated with salmonid harvest. Burgner et al.

89
90 NOAA Technical Report #105

(1982) and Burgner and Meyer (1983) showed that

temperatures below 15° C are often found within the

squid fishing area during each month of the fishing

season. They also noted that the 12° C isotherm extends into the zone during most months of the

fishery in cooler years and, more generally, that salmonid interceptions are potentially significant owing

to the large variability in SSTs. Ignell (1987) .documented the repeated occurrence of cold water

incident to the fishing area during recent years and

identified the importance of mesoscale variability in

SST for introducing cold water (11° C) several degrees south of the northern boundary during some

years. Because of interannual variability in oceanographic conditions in this region (Davis 1976), 19nell

(1987) concluded that the use of climatological values in the establishment of a squid driftnet regulatory

regime should be reexamined and an in-season management system explored.

Additional data from driftnet operations located

within the squid regulatory area have been collected

in recent years, allowing a direct assessment of the

potential for salmonid harvest by driftnet fishermen

operating near the northern boundary of the fishery.

In this paper, encounter and catch rates are calculated for salmonids from research vessel operations

located near the northernmost one-half degree latitude of the northern border. Also, differences

between catch rates in the eastern and western portions of the regulatory area are evaluated as a

function of SST.

Methods _

Data from 17 research cruises (vessels from Canada,

Japan, Taiwan, and the Republic of Korea (ROK) )

conducted between 1983 and 87 were used for the

analyses (Table 1). Twenty-seven driftnet operations

from the 17 cruises were located within the study

area-defined as the northernmost one-half degree

latitude of the 1987 Japanese squid regulatory areaand used for the analyses. Three additional

operations were included in the analyses because of

their close proximity (within 2 minutes of latitude) to

the study area.

Most of the research vessels deployed a combination of commercial and research drift net. Commercial

drift net was monofilament with stretched mesh sizes

of 110-130 mm on the Japanese vessels, 60-120 mm

on the Taiwanese vessel, and 115 mm on the Canadian vessel. Research drift net was either monofilament or multifilament, consisting of mesh sizes

designed to provide non-selective (i.e., constant

catchability) sampling for the target species (Takagi

1975). When catch and effort data from either very

small (33 mm) or very large (197 mm) mesh were

recorded separately, these data were excluded from

analyses because of the greatly reduced catchability

of salmonids. Therefore, the stretched mesh sizes of

research drift net included in the analyses were 48-

157 mm on Japanese vessels, 120-180 mm on the

Taiwanese vessel, and 33-115 mm on the ROK vessel.

Although length of net per set varied among operations (0.45-7.7 km), most of the 30 sets used about 5

km of drift net for a total deployment of 128.3 km of

drift net.

Data were divided into two geographical regions

(170° E to 175° Wand 145 W to 175° W, hereafter

called the western and eastern region, respectively)

according to similarity of oceanographic characteristics. Within these two areas, the data were further

divided into three SST intervals (<13° C, 13-15° C,

and ~15° C).

Table 1

The Canadian, Japanese, Taiwanese, and Republic of Korea (ROK) research vessel cruises that provided data for

analyses of salmonid catch as a function of sea surface temperature and geographical area.

Year of cruise

Nation

Taiwan

ROK

Canada

Japan

Japan

Japa~

Japan

Japan

Vessel name

Hai Kung

Pusan 851

Ricker

Oshoro Maru

Hokusei Maru

1waki Maru

Shoyo Maru

Kuromori Maru 38

1983

x

1984

x

x

1985

x

x

1986

x

x

x

x

x

x

1987

x

x

x

x

x

x
Average catch rates were computed for each of the

six strata. Catch rates were calculated by dividing total number of salmonids caught by total length of

drift net deployed.

For each strata, salmonid encounter rates were

computed as the proportion of driftnet operations

encountering salmonids (hereafter called the

uncorrected method). Because this estimate assumes

that the probability of salmonid incidence is unrelated to length of drift net deployed, two alternative

section-based estimates were developed. 1) Assume

that rfi.sh are randomly distributed in a drift net consisting of n driftnet sections (50-m segment) and the

probability of finding exactly m cells empty is Pm( r,n).

This is a variation of the classical occupancy problem

in probability theory (Feller 1968). The expected

number of empty cells is (Hamming 1991):

n (m-l) E(m) = Im' Pm'(r. n) =--n-l m

where the probability of salmon incidence per net

section (hereafter called salmonid incidence) is computed as 1 - E( m) / n. 2) Let q be the probability that

no salmonids are caught in a net section and xn the

total catch in n net sections. If catches in sections are

independent,

P[Xn= 0] = qn, P[Xn> 0] = 1 _ qn.

Let xn,(l), X n2 (2), ... x n/ KJ , be the results of K operations in a stratum. Define Yk as 0 if Xnk = 0, or 1 if Xnk

> 0; Zk as 0 if Xnk > 0, or 1 if Xnk = O. Then the likelihood function ofY1, Y2' •••, Yk and ZI' Z2' •••, Zk is

The two terms in the likelihood function identify the

probability of no catch and positive catch, respectively, in the k'h driftnet operation. Setting the partial

derivative of the log-likelihood with respect to q equal

to 0, gives

Solving for q provides a maximum likelihood-based

measure ofsalmonid incidence (P = 1 - q) that accounts

for differential net lengths between driftnet operations.

Computations of maximum likelihood estimates are restricted to three strata as they require both zero and

non-zero values ofsalmonid incidence within a stratum.

For each of the section-based methods, salmonid

encounter rate for a 5-km length of net can be computed from the incidence rate as

Ignell: Zonal Variability in Salmonid Driftnet Catch Rates 91

Encounter Rate = 1 - (1 _ P) 100.

Results _

Three hundred twenty-seven salmonids were encountered in 11 of 30 driftnet operations, resulting in an

overall catch rate of 0.127 salmonids per 50 m of drift

net (see Appendix Table). Most of the operations

(84%) occurred in sea surface temperatures of 15° C

or less. Chum salmon (0. keta) and coho salmon

(0. kisutch) were the predominant species caught,

comprising 60% and 34%, respectively, of the total

salmonid catch. The remaining salmonid catch consisted of chinook salmon (0. tshawytscha), pink

salmon (0. gorbuscha), and steelhead trout

(0. mykiss). In the area west of 175° W, almost 88% of

the chum were caught during August and September

although over 67% of the fishing effort (km) occurred in June and July. Conversely, 99% of the coho

were caught during June and July.

Ten of 19 (53%) gillnet operations in the northwestern portion of the squid fishing area encountered

salmonids compared to 1 of 11 (9%) operations

(uncorrected rates) in the northeastern area. This

5.8-fold difference was only partially explained by a

1.7-fold difference in the average length of drift net

deployed between areas (5.06 km per operation in

the western area versus 2.92 km per operation in the

eastern). Regional differences in uncorrected encounter rates were still prevalent when the data were

stratified further by SST interval (Table 2). Encounter rates were consistently higher in the western

region: 100% versus 25% in the western and eastern

regions, respectively, for driftnet operations in SSTs

<13° C; 40% versus 0% in SSTs of 13-15° C; and 25%

versus 0% in SSTs ~15° C.

Encounter rates computed from maximum likelihood-based incidence rates applied to the average

length of drift net deployed per strata were similar

to those of unweighted operation-based method

(Table 2). Encounter rates standardized to a 5-km

length drift net were estimated at 38% and 26% for

the western region SSTs of 13-15° C and ~15° C respectively, and 36% for eastern region SSTs of

<13° C.

Salmonid encounter rates estimated by the occupancy-based measure, however, were too high in two

of four strata. This overage was apparently caused by

the effect of heterogeneous incidence rates within a

strata, or non-random distribution of catch within a

driftnet operation, or both. Occupancy-based encounter rates were independently estimated for each

driftnet operation, then averaged across driftnet operations within each strata. Estimates of encounter
92 NOAA Technical Report #105

Table 2

Driftnet catch and effort stratified by region and sea surface temperature (SST) interval. Three estimates of salmonid

encounter rates are also presented (see text for description of estimates). Data are from research driftnet operations

located in the northernmost one-half degree latitude of the Japanese squid driftnet fishing area, 1983-1987.

1700 Eto175°W 145°Wto 175°W

SST interval (0C) SST interval (OC)

<13 13-15 >15 <13 13-15 >15

Catch (no.)

Coho 43 171 1 0 0 0

Chum 14 67 0 3 0 0

All salmon ids 74 248 1 4 0 0

Fishing effort

Number of operations 5 10 4 4 5 2

Total effort (km) 22.8 54.2 19.2 12.7 17.8 1.6

Average effort (km) 4.6 5.4 4.6 3.2 3.6 0.8

Encounter rate (%)

Uncorrected 100 40 25 25 0 0

Maximum likelihood

(strata average) 41 25 24

(standardized-5 km) 38 26 36

Occupancy (5 km) 100 ~OO 24 84

-----

rates for each strata, therefore, did not minimize between operation error variability (which the maximum likelihood method did minimize). Simulations

of alternative occupancy patterns showed this

overage could be reduced if fish were distributed

contagiously. Such a distribution would mean that

the assumption of random occupancy of net sections

was not met by the data. and that salmonid incidence

estimates based on the occupancy method would be

high.

The regional differences in salmonid incidence

were reflected in catch rate differences. Catch per

tan (50-m panel of drift net) of salmonids, hereafter

called CPUE, in the western and eastern regions was

0.168 and 0.006, respectively, representing a 28-fold

difference between regions. Salmonid CPUE stratified by SST interval (<13° C, 13-15° C, and >15° C)

was 0.162, 0.229, and 0.003 for the western region,

and 0.016, 0.0, and 0.0 for the eastern region. In the

western region. coho were the dominant species

caught when SST was <13° C (CPUE of 0.094 for

coho versus 0.031 for chum), whereas chum were the

dominant species from 13 to 15° C (CPUE of 0.164

for chum versus 0.062 for coho).

Discussion . _

Analyses of 1983-87 Japanese, Taiwanese, Canadian,

and ROK research vessel data show a zonal cline in

salmonid incidence and catch rates when the data

are stratified by SST interval and limited to the

northernmost portion of the squid fishing area.

Here, at the southernmost limit of salmonid habitat,

driftnet operations within a given range of SSTs are

more likely to encounter salmonids in the western

than in the eastern portion of the fishing area.

Regional differences in salmonid abundance may

contribute to the observed geographical variation in

catch rates. An analysis of 1955-1960 U.S. research

vessel data showed that the abundance of salmon was

greatest in the western North Pacific and in the

Bering Sea (Manzer et al. 1965). Recent data, although extensive for the western subarctic waters,

cover very little of the eastern waters, thereby limiting comparisons of salmonid abundance between

these areas.

Because the magnitude of regional differences in

catch rates is so great, the small number of driftnet

operations used in these analyses (and possible sampling biases) add variability to the 1983-87 data but

do not likely affect the conclusions. Any sampling

bias in the data probably reduces, rather than increases, catch rate differences. Sampling dates in the

western region occurred mostly towards the end of

the month. This sampling bias, however, is not apparent in the eastern region. Because salmonid harvest

within the squid fishing area is most pr.obable du.ing

the first several days of each summer month (Ignell

1987), catch rates for the western region are likely

biased downwards when compared to eastern catch

rates.
Ignell: Zonal Variability in Salmonid Driftnet Catch Rates 93

Figure 1

Frequency distribution of the number

ofsalmonids caught per 5 km of driftoet

effort, stratified by the stretched mesh

size of the drift net. Data from the Japanese RV Hokusei Maru, 1983-1987

cruises.

82 93 106 121 138 157

Mesh Size (mm)

48 55 63 72

O--"I"""".........""'l""-

100

280..,----------------

Differences in average mesh size between research

vessel cruises also cannot explain geographical differences in catch rates. Vessels operating in the western

area with mesh sizes ~1 05 mm encountered salmonids much more frequently than vessels operating

in the eastern area with smaller mesh sizes <105 mm.

The opposite would be expected based on Japanese

research vessel data that show CPUE is largest for

mesh sizes of 93 mm (Fig. I).

Geographical differences in the CPUE/SST relationship may be explained by physical oceanographic

differences between the two regions because watermass structure is an important factor affecting the

ocean distribution of salmonids (Favorite and

Hanavan 1963; Murata 1987). Although SST isotherms are almost strictly zonal across the North

Pacific Ocean, the subarctic frontal zone angles

northward 4° to 6° of latitude to the east (Levine and

White 1981). Because of this northward angling, subarctic water intrusions, as defined by the subsurface

thermohaline structure, are less likely to occur in the

eastern portion. As a result, salmonid catch rates in

the eastern region are reduced compared to the western region where intrusions of subarctic water are

more frequent.

The salmonid incidence data show that there is a

high probability of salmonid harvest (primarily chum

and coho salmon) when SSTs are <13° C, and very

low incidence when SSTs are ~15° C. These results

generally reflect those of Ogura and Takagi (1987):

waters inhabited by chum and coho salmon usually

have SSTs <13° C; however, their catch rates were several times that of data from the western region and

approximately an order of magnitude greater than

the eastern region. For instance, catch per tan of

chum salmon in the western squid regulatory area

was 0.03 and 0.14 (for SSTs of 10.5-13° C and 13-

15° C), respectively, as compared to Ogura and

Takagils (1987) estimate of about 0.5 for driftnet operations in SSTs of 11-12.9° C. Catch per tan for

coho salmon were less disparate between the two data

sets, ranging between 0.2 to 0.4 for SSTs of 11-

12.9° C (Ogura and Takagi 1987) as compared to

0.12 in my analysis.

Although catch rates presented in this paper are

imprecise owing to a limited number of observations,

the large differences in catch rates between the two

data sets are likely real and may stem from geographical differences. Ogura and Takagi's (1987)

data were obtained throughout a~broad portion of

the North Pacific Ocean, whereas data used in the

present study were constrained to operations within

the Japanese squid regulatory area. Thus, although

neither data set associated catch data with water-mass

type (subarctic domain versus subarctic frontaL

zone), the proportion of driftnet operations in subarctic water is probably highest for Ogura and

Takagi's (1987) data.

The above sampling differences are important because marine organisms are distributed according to

environmental conditions needed for successful development (Uda 1961). In the epipelagic marine

community of the North Pacific Ocean, these environmental conditions are characterized in terms of

water-mass properties (rather than SST) which reflect

the oceanographic characteristics important to the
94 NOAA Technical Report #105

species complex of a particular region (Shimazaki

1986). Salmon are part of the fish community associated with subarctic water; they are seldom found

south of subarctic waters and then only withir: the

northern portion of the subarctic frontal zone

(Shimazaki 1986; Murata 1987). Thus, catch rates

from driftnet operations located in subarctic waters

should exceed those from driftnet operations located

within the subarctic frontal zone, even if SSTs are

similar between areas.

Acknowledgments _

I am grateful to Jerome J. Pella of the Auke Bay Laboratory for his advice on approaches for estimating

salmonid encounter rates in variable length drift nets

when section-level data are missing.

Citations _

Burgner, R.L., and W.G. Meyer.

1983. Surface temperatures and salmon distribution relative

to the boundaries of the japanese drift gillnet fishery for

flying squid (Ommas/rephes bar/rami). In t. North Pac. Fish.

Comm. (INPFC) Document, Fish. Res. Inst., Univ. Washington, Seattle, 35 p.

Burgner, R.L., R.W. Mercer, and R.L. Major.

1982. Surface temperatures and salmon distribution relative to the japanese driftnet fishery for flying squid

(Ommas/rephes bar/rami). INPFC Document, Fish. Res.

Inst., Univ. Wash., Seattle, 22 p.

Dahlberg, M.L., and M.F. Sigler.

1987. The distribution and abundance of squid and salmon

in the central north Pacific Ocean during the summer of

1986. INPFC Document, NMFS, NOAA, Auke Bay, 18 p.

Davis, R.E.

1976. Predictability of sea surface temperature and sea level

pressure anomalies over the North Pacific Ocean. j. Phys.

Oceanogr. 6:249-266.

Favorite, F., and M.G. Hanavan.

1963. Oceanographic conditions and salmon distribution

south of the Alaskan Peninsula and Aleutian Islands. Int.

North Pac. Fish. Comm. Bull. 11:57-72.

Feller, W.

1968. An introduction to probability theory and its applications, Vol. 1. john Wiley and Sons, NY, 509 p.

Fisheries Agency ofjapan.

1982. Squid drift gillnet fishery. INPFC Document, Fisheries Agency ofjapan, Tokyo,japan 100, 9 p.

Hamming, R.W.

1991. The art of probability for scientists and engineers. Addison-Wesley, Redwood City, 344 p.

Harris, C.K., and G.A. Kautsky.

1987. Distributional overlap of salmonids and flying squid

(Ommas/rephes bar/rami) in the western and central north

Pacific Ocean, as indicated by analysis of japanese salmon

research vessel catches and incidental catches, 1972-

85. INPFC Document, Fish. Res. [nst., Univ. Wash., Seattle,

54 p.

Ignell, S.E.

1987. Anomalous oceanographic conditions in the central

north Pacific Ocean, 1986-87. INPFC Document, NMFS,

NOAA, Auke Bay, 17 p.

LeBrasseur, R., B. Riddell, and T. Gjernes.

1987. Ocean salmon studies in the Pacific Subarctic boundaryarea. 1 PFC Document, Dep. of Fish. and Oceans, Bio.

Sci. Branch, Pacific Biological Station, Nanaimo, B.C. V9R

5K6, 16 p.

Levine, E.R., and W.B. White.

1981. Large-scale synoptic thermal fronts in the mid-latitude

north Pacific from 1976-78. j. Geophys. Res. 86(C7) :6567-

6579.

Manzer,j.!., T. Ishida, A.E. Peterson, and M.G. Hanavan.

1965. Salmon of the North Pacific Ocean. Part V: Offshore

distribution of salmon. Int. North Pac. Fish. Comm. Bull.

15:1-452.

Murata, M.

1987. Report on fishing survey on flying squid by the drift

gillnetters Shoyo-maru, Kuromon-maru No. 38, and Kanki-maru

No. 58 in the north Pacific in 1986. INPFC Document,

Fisheries Agency ofjapan, Tokyo,japan 100,20 p.

Ogura, M., and K. Takagi.

1987. Differences in area of distribution between salmon

and flying squid based on data collected by japanese salmon

research vessels, 1983-1986. INPFC Document, Fisheries

Agency ofjapan, Tokyo, japan 100, 7 p.

Shimazaki, K.

1986. Distribution of the pelagic fish community around the

Subarctic boundary in the North Pacific Ocean. lnt. North

Pac. Fish. Comm. Bull. 47:247-264.

Takagi, K.

1975. A non-selective salmon gillnet for research operations. Int. North Pac. Fish. Comm. Bull. 32:13-41.

1983. Differences in area of distribution between salmon

and flying squid. [NPFC Document, Fisheries Agency of

japan, Tokyo,japan 100, 10 p.

Uda, M.

1961. Fisheries oceanography in japan, especially on the

principles of fish distribution, concentration, dispersal and

fluctuation. Calif. Coop. Oceanic Fish. Invest. 8:25-31.
IgneU: Zonal Variability in Salmonid Driftnet Catch Rates 95

Appendix Table

Salmonid catch data from 30 research vessel operations located near the northernmost one-half degree latitude of the

Japanese squid driftnet regulatory area, 1983-87. a

Avg.

Tans of net mesh Net Catch in numbers

size Igth.

Vessel Date Lat. Long. SST Comm. b Res. c (mm) (km) Chum Coho Pink Chinook Trout Total

Hokusei 7-22-83 41°31'N 170°01 '£ 11.0 80 30 110 5.5 5 6 II

Ma7u 7-29-83 41°32'N 175°33'£ 13.5 80 30 110 5.5

7-21-84 4J030'N 170°00'£ 15.0 80 30 llO 5.5

7-27-84 41°29'N 175°45'£ 14.4 80 30 110 5.5

7-21-85 41°31'N 170°02'£ 15.0 80 30 110 5.5

7-27-85 41°29'N 175°27'£ 15.7 80 30 llO 5.5

7-28-86 41°30'N 175°55'£ 16.0 70 30 llO 5.0

7-28-87 41°31 'N 174°59'£ 14.5 50 30 108 4.0

Oshoro 6-13-84 39°30'N 180°00' 11.1 48 30 107 3.9 14 14

Maru 6-16-86 400 00'N 179°59'W 14.8 60 40 III 5.0 14 65 79

6-14-87 400 02'N 180°00' 16.7 50 40 108 4.5 I I

Iwaki 7-13-85 41°58'N 172°10'£ 10.7 102 30 108 6.6 23 17 41

Maru

Kuromori 7-20-86 41°30' 180°00' 13.0d 34 50 100 4.2

Maru 38 8-27-86 43°30' 175°00'£ 16.0d 34 50 100 4.2

Shoyo 7-15-86 42°00'N 1500 00'W 14.5d 34 50 100 4.2

Maru 8-14-86 44°00'N 165°00'W 14.0d 34 50 100 4.2

7-11-87 42°00'N 1500 00'W 11.7 34 50 100 4.2

Hai Kung 8-05-86 44°00'N 173°00'£ 14.6 171 99 5.6 40 41

8-13-86 44°00'N 177°00'W 14.8 172 97 5.7

9-14-86 46°00'N 175°00'W 13.1 198 36 107 7.7

9-18-86 46°00'N 171°00'£ 13.2 198 36 107 7.7 124 2 127

8-14-87 43°59'N 176°46'W 11.7 150 102 5.4 5 5

Pusan 851 7-26-86 41°58'N 1700 00'W 14.9 10 96 0.5

7-28-86 41°30'N 172°00'W 16.0 16 94 0.8

7-31-86 42°00'N 175°00'W 17.5 16 94 0.8

7-28-87 42°00'N 1700 00'W 14.0 23 79 1.2

8-06-87 44°00'N 178°40'£ 11.9 27 74 1.4 3 3

Ricker 6-04-87 39°38'N 161°04'W 11.9 54 ll5 2.6 3 4

6-23-87 39°41 'N 1600 37'W 11.7 62 115 3.0

6-25-87 39°48'N 15T25'W 11.7 60 115 2.9

Total: 195 111 17 2 2 327

a Source: Data record of Oceanographic Observations and Exploratory Fishing, The Faculty of Fisheries, Hokkaido University, Nos.

27, 28, 29, 30, and 31; Murata (1987); Technical Report of National Fisheries Research and Development Agency, Pusan, Republic

of Korea (ROK), Nos. 68, 76; Cruise Report of the RV Hai Kung, HK-1986, HK-1987, Taiwan Fisheries Research Institute, Keelung,

Taiwan, Republic of China; Lebrasseur et aI., (1987); Cruise Report of the Shoyo Maru in the central North Pacific Ocean, 26 June

through 28 August 1987 by Michael Johnson, U.S. Dep. of Commerce, National Marine Fisheries Service, National Marine Mammal

Laboratory, Seattle, Washington.

b Mesh sizes varied by research vessel; Japanese commercial mesh sizes were 110-130 mm, Taiwanese commercial mesh sizes were

60-135 mm, and Canadian commercial mesh sizes were 115 mm.

C Only includes "C" type net for Japanese operations (48-157 mm). Mesh sizes for Taiwanese research drift net ranged from

120-180 mm, and for ROK research drift net, from 33-115 mm.

d SST data interpolated from contour plot.

The Fisheries for

Neon Flying Squid (Ommastrephes Bartrami)

in the Central North Pacific Ocean

STEVE IGNELL

Auke Bay Laboratory

A laska Fisheries Science Center

National Marine Fisheries Service

National Oceanic and Atmospheric Administration

11305 Glarier Highway, juneau, Alaska 99801-8626

ABSTRACT

During the 1970s, the increased exploitation of squid stocks near Japan and reduced

harvest opportunities or poor market conditions affecting other distant-water fisheries

led Japan, Taiwan, and the Republic of Korea to fish the neon flying squid (Ommastrephes

bartrami), an under-utilized food species abundant across the North Pacific Ocean. Japanese jigging vessels pioneered the fishery in 1974, but were displaced by the more

efficient driftnet vessels ofJapan in 1978, the Republic of Korea in 1979, and Taiwan in

1980. Each year, over 750 driftnet vessels fish from April to December. Each vessel deploys up to 60 km of drift net nightly. Vessels fish independently or within a multi-vessel

array where nets are set along an east-west direction and are spaced 2-4 km apart along a

north-south axis. Total catch ranges from 200,000 to 300,000 t. In recent years, vessel

numbers have stabilized or, in the case of the Japanese fleet, have decreased. Annual

effort (i.e., the total amount of drift net deployed annually), however, has increased

owing to increased vessel sizes, length of drift net deployed, and fishing season. As a

result, catch per vessel day has remained constant or increased in recent years, while

catch per unit length of net has decreased in the Taiwanese and Republic of Korea

fisheries. The squid are processed by separating the mantle from the head; mantles are

frozen. Some incidentally caught species (such as tuna, billfish, yellowtail, and pomfret)

are also retained and processed. Japanese vessels fish under a complex set of fishery

regulations designed to, inter alia, limit competition with jigging vessels in the western

Pacific, limit total fishing effort, and minimize the incidental (and prohibited) catch of

salmonids. Korean and Taiwanese vessels are less regulated and primarily fish west of the

Japanese driftnet fleet.

Introduction _

The neon flying squid (Ommastrephes bartrami)

driftnet fisheries ofJapan, Taiwan, and the Republic

of Korea (ROK) in the North Pacific Ocean are in

combination the largest of the pelagic driftnet fisheries. Each year, over 750 driftnet vessels fish from

April to December. Each of these vessels deploys up

to 60 km of drift net nightly at speeds of about 10

knots, retrieving the gear at speeds up to 5 knots.

Their efficiency has created financial difficulties for

vessels using more traditional and less efficient harvesting methods such as jigging (Araya 1983);

however, it has enabled the profitable exploitation

of an important and previously under-utilized food

species.

Drift nets, although highly efficient harvesting devices, do not discriminate well between species. Most

non-target species too large to pass through the mesh

are entangled and killed or drop out before being

brought aboard, creating waste and potential economic loss (Major 1982). Though of little

commercial value, some of these species are major

components in the North Pacific Transition Zone

ecosystem (Shimazaki 1986), and/or have cultural

value (e.g. marine mammals, seabirds, and some

fishes) .

97
98 NOAA Technical Report #105

These factors have led in recent years to increased

concern about the possible impact of incidental

catches by squid driftnet fisheries on North Pacific

Ocean marine resources (Eisenbud 1985). Incomplete information about fishery impacts has

increased this concern. Although 11 years have

passed since the inception of the fisheries, estimates

of the magnitude of incidental take are still difficult

to obtain. The dynamics of the fisheries (e.g.,

amount of gear deployed) are also not well known.

In particular, there is difficulty in defining a unit of

fishing effort. Fishing effort is reported three ways:

number of vessels fishing, days fished, or the number

of units of gear fished. These three effort measures,

however, have not been available for each fishery,

and may not be reported consistently within a fishery

or among fisheries (e.g., one unit of effort may be

50 m of drift net for the ROK fishery, but 42 to 105 m

for the Japanese fishery [Ignell et al. 1986]). As a

result, calculations of catch per unit effort (CPUE)

depend upon the type of effort measure used, and

may not be comparable between fisheries.

This paper provides a summary of the North Pacific squid driftnet fisheries ofJapan, the ROK, and

Taiwan as of 1989. The following sections on each of

these national fisheries cover fishery development,

characteristics, and regulations; gear and vessel characteristics; and fishing methods. The last section is

devoted to an overview of the fisheries, including a

discussion of recommended catch levels for flying

squid given historic catch and effort.

Japanese Squid Driftnet Fishery _

Squid have long been important to the economy and

diet of the Japanese. Dried squid was a major trade

item as early as the 1600s, becoming even more important in 1864 when Chinese markets opened to

this product (Osako and Murata 1983).

After World War II, high unemployment, low labor

cost (due to Japanese troops returning home), and a

food supply shortage resulted in a great expansion of

the fishery for the Japanese common squid (Todarodes

pacificus); little capital was needed for its growth compared with other industries (Court 1980; Osako and

Murata 1983). Catches of common squid rose rapidly,

from about 100,000 t in 1945 to over 500,000 t in

1951. The catches were of the winter group (common squid are divided into three groups according

to spawning time: winter, summer, and autumn),

which had the largest biomass and was primarily

caught along the Pacific coast ofJapan.

Until the late 1950s, most squid were caught by

hand with jigging gear. With the development of automatic jigging machines and fishing lamps, fishing

power increased dramatically (Osako and Murata

1983). After a record catch of 670,000 t in 1968, winter group catches fell to below 10,000 t by 1976. This

decrease prompted the exploitation of the autumn

group on new, offshore, fishing grounds during the

early 1970s (Chikuni 1985). These grounds, primarily

in the Sea of Japan, immediately suffered heavy exploitation, and catches decreased after only a few

years of fishing (Okutani 1977).

As fishing grounds shifted offshore, fishing vessels

became motorized and the number of large vessels

increased. The emergence of large jigging vessels and

the subsequent decline in common squid prompted

fishermen in 1974 to target neon flying squid (hereafter called flying squid) in the northwest Pacific

Ocean east of Hokkaido and northeast of Honshu

(Osaka and Murata 1983).

Driftnet vessels first began fishing for flying squid

in September 1978. Over 170 Japanese salmon

driftnet vessels (displaced from their traditional

salmon fishing grounds owing to reduced harvest

quotas in 1978) were the first to use drift nets for

flying squid (Herrfurth 1988), but were soon followed by many squid boats (Court 1980). A classic

example of gear conflict developed. Jigging proponents argued that their established, economicallydependent fishery gave them a historic right to the

flying squid resource. Furthermore, they opposed

driftnetting because it was too efficient (2-4 times

more than jigging [Araya 1983]), was wasteful because of dropouts, created conservation problems,

and was often used as a cover for illegal salmon

(Oncorhynchus spp.) fishing (Court 1980).

Fishing Regulations

On 1 January 1979, the Fisheries Agency of Japan

(FAJ) addressed this conflict by prohibiting driftnets

west of l70oE, south of 200 N (the area fished by jigging vessels). Continued financial losses by jigging

vessels-in part due to competition from the large

number (up to 1000) of Japanese driftnet vessels

(Court 1981)-resulted in the 1981 limited-entry

program. This program established regulatory measures to minimize the incidental catch of salmonids

(Court 1981). These regulations as outlined in FAJ

(1982) set a 7-month fishing season from June

through December, a northern boundary designed to

avoid the incidental catch of salmonids, and an eastern boundary at 145°W (Fig. 1). The northern

boundary was set to coincide approximately with the

long-term monthly average of the 15°C surface

isotherm (I PFC 1982), and to change monthly.
Ignell: The Fisheries for Neon Flying Squid 99

20N

SON

140W 120W

."

tt

160E 160W

eoN

"... ........• ..~. ......,,"

SON

•

~. 44N

39N 40N 40N

Figure 1

Time and area regulations for the Japanese and Taiwanese squid driftnet fisheries in the north Pacific Ocean (Fisheries Agency

ofJapan 1985b). For both fleets, fishing is prohibited east of 145°W, and north of 400 N in June and December, 42°N inJuly and

November, 44°N in August and October, and 46°N in September. ForJapanese vessels, fishing is prohibited west of 170°£, and

for Taiwanese vessels, north of 39°N and west of 170°£.

Japanese scientists felt this regulation would enable

the squid driftnet fishery to avoid incidentally catching salmon (Takagi 1983).

Under the limited entry established by the FAJ, two

types of permits for vessels between 50 and 500 gt are

issued, one allowing 7 months of operation (1 June

to 31 December), the other, 4 months (l August to

30 November). Other regulations require net mesh

sizes between 100 and 135 mm to conserve squid resources, submission of a catch-and-operation report

by vessel operators after a fishing trip, special identification coloring and permit display on the vessels,

non-retention of salmonids, and a limit of three ports

per vessel for landing catch. Fishing vessels are also

required to notify the FAJ when approaching and

leaving the fishing grounds at 170oE. When navigation beyond the northern and eastern boundary is

unavoidable, ijshermen are required to report the estimated course and itinerary one day in advance.

Fishery Characteristics

japan's annual catch of flying squid by the driftnet

fishery increased from 46,308 t in 1978 to a peak of

216,000 t in 1983, then decreased slightly to about

208,000 t in 1987 (Table 1). Since the early to mid1980s, there has been a steady decrease in vessel

numbers, but an increase in permitted fishing time

and in vessel size (Table 2). Because larger vessels

fish 2.67 times more gear than smaller vessels (M.

Nakamura, FAJ, Tokyo, Japan 100, pers. commun., 10

March 1989), the increase in fishing time and vessel

size has likely offset the decrease in vessel numbers

in terms of the total amount of drift net deployed

annually.

The Japanese squid driftnet fishing season begins

in June and lasts through December. More than 42%

of the average annual fishing effort (vessel days) and

50% of the average annual catch since 1983 has oc-
100 NOAA Technical Report #105 _

Table 1

Catch and fishing effort statistics of the Taiwanese C~epublic of China-ROC) a, Japanese b, and Republic of Koreac (ROK)

squid driftnet fisheries in the North Pacific Ocean, 1977-88. Korean data from sampled vessels are given in parentheses.

NA = not available.

Driftnet vessels Vessel days Driftnet catch (t)

Year ROC japan ROK ROC japan ROK ROC japan ROK

1977

1978 NA NA 46308

1979 NA 3 (I) NA lOR (36) 47652 360 (120)

1980 12 A 14 (9) NA NA 1064 (684) 908 119000 4693 (3017)

1981 44 534 34 (9) NA NA 5190 (1374) 10719 104000 22901 (6062)

1982 88 529 60 (56) 5636 33085 6825 (6374) 2186/l 158722 23738 (21364)

1983 101 515 99 (77) 6505 32685 12862 (10004) 14257 215778 48513 (37732)

1984 146 505 III (105) 11389 32555 17297 (16362) 27553 123719 52266 (49441)

1985 ]24 502 125 (104) NA 35132 19799 (16473) 21807 197795 85495 (7] 132)

1986 1I0 492 1I7 (107) NA 35549 2051H (18764) ]3887 152226 64539 (59023)

]987 87 478 140 NA 6635 296]3 NA ]8578 208319 NA

1988 166 463 150 A .\ 3]998 NA NA 157773 NA NA

-------

a 1980-81 data from Liu (] 984); ]982, ] 983, and 1984 data from 1982-1984 editions of Statistics of Deep Seajigging and Gill Net Fishery

of Squid in Northwest Pacific Ocean, Institute of Fishery Biology of :\1inistry of Economic Affairs and National Taiwan University; 1986

data from Report of the November 1988 U.S.-Taiwan Bilateral Meeting on Assessment of North Pacific Fisheries Resources, 5-6

November 19R8. at Institute of Oceanography, National Taiwan Cniversity, Taipei, Taiwan; 1983-87 data from Tung (1988); and 198]-

88 data from CCNAA (unpublished data, previously cited).

b Catch and etlort data from Masayuki Komatsu, Fishery Agency ofjapan, Tokyo, pers. commun., Sept. 1986; Fishery Agency ofjapan

(1982, 1984, 1985a, 1986, 1987, 1988, 1989); and Fisheries Yearbook, 1986, Ministry of Agriculture, Forestry, and Fisheries,japan.

C Data from NFRDA (1988). The catch and fishing effort statistics (or the entire fleet are computed by using sample data divided by the

sampling fraction.

Table 2

Number ofJapanese driftnet vessels categorized by vessel size and permit type that are authorized to harvest squid in the

North Pacific Ocean 1981-1988°.

Permit type Vessel a class (gt) b

Year 59-100 100-500

]981 37] 163

1982 326 203

1983 285 230

1984 265 240

1985 259 243

1986 237 255

1987 209 269

1988 202 26]

4-momh 7-momh

418

426

478

439

Total

Vessels

534

529

515

505

502

492

478

463

a Data from Snead et aJ. (1988). Herrfurth (] 988), and the Regiunal Fisheries Attache, American Embassy, Tokyo.

b gt = gross tons.

C All but three permits used by 59-100 gt vessels.

d All but one permit used by 59-]00 gt vessels.

curred in July and August (Table 3), About 85% of

the mean monthly fishing effort (vessel days) in July

and August takes place in the northern 2° latitude of

the permitted fishing area, producing about the same

mean percentage of the monthly catch each year

(Table 4).

Five different vessel classes operate in the Japanese

squid driftnet fishery (Table 5), Entry time to the

fishery and fishing season length vary by vessel class

because most vessels are committed to other fisheries

(such as salmon. squid jigging, or groundfish) during

part of the year. Vessels from the mothership and
IgneU: The Fisheries for Neon Flying Squid 101

Table 3

Fishing effort and catch per unit effort stratified by month for Taiwanese a, japaneseb, and ROKe squid driftnet fisheries

operating in the North Pacific Ocean, 1983-1987. Fishing effort is given in vessel days for the Taiwanese and Japanese

fisheries, and in pieces (50 m) of drift net for the ROK fishery.

Fishing effort (vessel days) Catch (t)/vessel day

Month 1983 1984 1985 1986 1987 1983 1984 1985 1986 1987

JAPAN

Jun 4348 4124 3664 4832 5001 6.98 4.47 6.57 5.75 7.80

Jul 5410 6580 5802 7599 6896 8.84 5.42 7.94 5.23 10.31

Aug 8137 7923 8072 8721 5697 7.65 3.60 6.20 4.50 7.72

Sep 5825 5895 6326 6049 5261 6.47 2.79 4.88 3.20 5.22

Oct 5100 4124 5956 3847 4207 4.08 2.86 4.02 3.14 3.65

Nov 2917 2940 4013 3396 1892 4.18 3.31 4.46 2.95 4.63

Dec 948 969 1299 1105 659 4.85 2.95 3.82 3.61 3.97

Total: 32685 32555 35132 35549 29613 X: 6.60 3.79 5.63 4.28 7.03

TAIWAN

Apr 9 208 0.69 1.38

May 284 734 125 0.92 1.53 1.46

Jun 633 1123 705 1.67 1.95 1.94

Jul 865 2393 1453 2.54 1.80 2.76

Aug 1499 2642 1743 2.47 2.14 2.43

Sep 1839 2714 1704 1.97 3.28 3.36

Oct 952 1559 881 1.17 3.25 3.38

Nov 424 16 24 0.49 3.75

Total: 6505 11389 6635 X: 2.19 2.42 2.77

Fishing effort (pieces of net) Catch (t)/net

Month 1983 1984 1985 1986 1987 1983 1984 1985 1986 1987

ROK

Apr 66 247 2.00 0.87

May 1147 2266 2.14 2.09

Jun 1981 2888 4.89 3.02

Jul 1954 2217 6.81 3.31

Aug 2266 2603 6.30 2.80

Sep 2614 2569 6.07 5.22

Oct 1860 2326 4.89 4.71

Nov 1346 1714 3.75 3.04

Dec 743 744 1.69 1.50

Jan 12 13 2.33 0.85

Total: 13989 17587 X: 5.08 3.36

a Data from 1983 and 1984 editions of Statistics of Deep Sea Jigging and Gill Net Fishery of Squid in Northwest Pacific Ocean. Institute of

Fishery Biology of Ministry of Economic Mfairs and National Taiwan University. Data (1987) from CCNAA (unpublished data, previously cited). .

b Data from Fisheries Agency ofJapan (1984, 1985a, 1986, 1987, 1988).

c Data (1985) from NMFS (1986). Data (1986) from NFRDA (l987b).

land-based salmon driftnet fisheries are the smallest,

and fish primarily during late summer and fall in

western waters. Vessels dedicated to squid driftnet

fishing are the only class that fish an entire season

(June through December).

Vessels fish independently or within multi-vessel arrays. Array fishing is a group of vessels coordinating

net deployment, usually in an east-west direction,

with vessels spaced at least 3.5 km apart along a

north-south axis (Tsunoda 1989). The organization is
102 NOAA Technical Report #105 _

Table 4

Catch (t), effort (vessel days), and the percentage of total monthly catch and effort within 2° of the northern boundary of

the Japanese squid driftnet fishery for July and August (the months of peak effort) and for the entire season (FAJ 1984,

1985a, 1986, 1987, 1988)

July August

Year Catch (%) Effort (%) Catch (%) Effort (%)

1983 34515 (72) 3959 (73) 51448 (83) 6404 (79)

1984 33741 (95) 6195 (94) 24675 (87) 6926 (87)

1985 43339 (94) 5431 (94) 41210 (82) 6625 (82)

1986 37850 (95) 7184 (95) 36359 (93) 7931 (91)

1987 59786 (84) 5677 (82) 34358 (78) 4039 (71)

Average 41846 (87) 5688 (88) 37610 (84) 6385 (83)

Table 5

Characteristicsa of vessel classes participating in the Japanese squid driftnet fishery in the North Pacific Ocean, 1987.

Fishing Grounds

<----------------~

<----------------->

<----------------->

<----------------->

<-------------------------~

Vessel

class

Mothership salmon

Landbased salmon

Trawlerjlongliner

Jigging

Dedicated

Avg. No. of

(t) vessels

96 42

80 80

350 49

345 39

300 172

170°£ 175°W 1600 W 145°W

Main

season

Aug-Nov

Aug-Nov

Jun-Sep

Jul-Oct

Jun-Nov

a Data from the Fisheries Agency ofJapan, Tokyo,Japan (unpub!. data).

not predetermined; it develops on the fishing

grounds. Arrays usually consist of vessels of si:nilar

class, because information is rarely exchanged between vessel classes on the fishing grounds.

Gear and Vessel Characteristics

The number of single panels (tans) of net that are set

nigh tly varies among vessels, and depends on weather

and other factors (Tsunoda 1989). Amount of net

fished is relative to vessel size; small vessels (<100

gross tons) set as few as 450 tans nightly, whereas

large vessels typically set 1200 tans nightly (M.

Nakamura, FA], Tokyo, Japan, pers. commun., 10

March 1989). Net dimensions are not standardized

and may be changing over time. In 1985,. the FA]

(l986b) reported a tan to be 72-90 m long wi th a

corkline and leadline shrinkage factor of 46.9% and

45%, respectively. Observations (by U.S. scientific observers and enforcement personnel in 1985 and

1986) indicated tans ranged from 40 to 105 m in

length (Ignell et al. 1986; Tsunoda 1989) and 9 to 10

m in depth. More recent information from Japanese

officials indicates that tans range in length from 36-

50 m (Snead et al. 1988). All nets are of similar

design-a corkline at the surface, monofilament

mesh stretched horizontally, and a leadline at the

bottom-with stretched mesh sizes typically ranging

from 115 to 120 mm. Single tans are tied and laced

together to construct "sections" of gear approximately 5.5 km long; two may be tied together to form

double-length sections. Most large vessels deploy at

least 8-10 net sections per night.

The Republic of Korea

Squid Driftnet Fishery _

Fishery Characteristics

The Republic of Korea (ROK) high-seas, driftnet

flying squid fishery began in 1979 as an exploratory

fishery (Gong 1985); three vessels fished from August

through October off the coast of ] apan (Table 1)
1,000

Ignell: The Fisheries for Neon Flying Squid 103

50

..... ..... • 0 .....

~. /////

o__ / ..... o~

---.----.---.---

..... 0 ..... .......... 0 .....

.....--- 0-

/

/

/

/

/

/

/

o ..... .....

>.

III

'C

I 800 .-<

tlJ

Ul

Ul

tlJ

" 600 <l)

0-

+J

tlJ

C

::1 400 ''';

O'l

.....

o

o 200 z

o

1980 1981 1982 1983 1984 1985 1986

30 C

+J

''';

C

;l

20 "

<l)

0-

..c

tJ

+J

10 B

o

Figure 2

Annual fluctuation of

fishing effort (open circle)

and catch per unit effort

(dark circle) for the Korean flying squid driftnet fishery in the north

Pacific Ocean, 1980-1986

(NFRDA 1988).

(NFRDA 1988). The fishery expanded the following

year when 14 vessels fished March through December, from the Japanese coast to 180° longitude. Since

then, the number of registered vessels has continued

to increase yearly: National Fisheries Research and

Development Agency (NFRDA) records show 99 licensed vessels in 1983 and 150 in 1988. Since 1980,

the fishing grounds have expanded eastward

(NFRDA 1988), reaching 155°W in 1986.

Korea's annual driftnet catch of flying squid increased from 360 t in 1979 to a peak of 85,495 t in

1985, then decreased to 64,539 t in 1986 (Table I).

Although the catch increase generally parallels the

increase in vessel numbers, CPUE (per 50 m of drift

net) has steadily decreased since the early 1980s to

the point where increased number of vessel days in

recent years has been offset by the decline in catch

rates (Table I; Fig. 2).

The ROK fishing season begins in late April near

35°N, 165°E. By May, most of the vessels have moved

eastward (NFRDA 1988), fishing from 1700 E to

155°W. Approximately half the fleet moves back westward (west of 1700 E) in July, the rest in August. The

westward movement continues during the year, with a

few vessels fishing near Japan's coast at season's end

in January.

Fishing is concentrated along a 4° band in latitude

that varies latitudinally by month (NFRDA 1988).

East of 1700E during the summer, ROK vessels are

generally located slightly south of the Japanese fleet

(FAJ 1987). About one-fourth of the total catch came

from east of 1700 E in 1985 (NFRDA 1987a), but that

proportion is undoubtedly greater in recent years owing to a gradual eastward shift in fishing location

(NFRDA 1988).

The vessel movements reflect the two distinct seasons and fishing grounds of the ROK fleet. "Western

grounds" are primarily between 1600 W and 1700 E,

and are fished late spring and summer. "Eastern

grounds" are between 143°E and 169°E, and are

fished from mid-August through December; squid

here are smaller but more abundant than those in

western grounds (Gooder 1989).

Vessels are organized into 16 fleets to ensure orderly conduct of the fishery (K.Y. Kim, fishery

Attache, Republic of Korea, Embassy of the Republic

of Korea, Washington, DC, pers. commun., 17 February 1989); fishing groups typically consist of 7-10

vessels whose captains attended the same fishing college (Gooder 1989). Within each fleet, one vessel is

selected to establish communication links among vessels themselves and with the Korean Deep Sea

Fishery Association (KDSFA). Observations from a

U.S. scientist aboard a ROK driftnet vessel in 1988

indicate that vessels of a fleet fish in an array pattern

and exchange information regarding fishing success

(Gooder 1989). The arrays consist of nets set in an

east-west orientation, spaced approximately 3.5 km

apart along a north-south axis, similar to those of the

Japanese fleet. Some vessels fish up to nine months

before returning to port; off-loading catch, replenishing supplies, and refueling are done at sea.
vessels must be licensed. Directives concerning the

non-retention of salmonids, the accounting for all

driftnets and vinyl products before and after each

cruise, and the proper disposal of vinyl products and

waste drift net are provided through licensing conditions or by KDSFA and governmental instruction.

Each year before vessels embark for the fishing

grounds, vessel captains must participate in an instruction program conducted by the NFRDA where

In an overview of ROK fishing regulations, Kim

(pers. commun. already cited) stated that ROK squid

driftnet vessels are jointly managed by the KDSFA (an

industry group representing most Korean fishing

companies) and by the government. Vessels are not

subject to time or area regulations, and there is no

limited-entry system established for the fishery. All

104 NOAA Technical Report #105 _

Fishing Regulations

Table 6

Composition of vessels according to gross tonnage for ROKa and Taiwanese b squid driftnet fisheries.

a National Fisheries Research and Development Agency ( FRDA) , Republic of Korea, 1988 (unpub!. data).

b Data from 1983 and 1984 editions of Statistics of Deep Sea Jigging and Gill Net Fishery of Squid in Northwest Pacific Ocean, and Tung

(1988) .

Figure 3

Trends in the amount (tans) of drift net set nightly by ROK

(upper graph) and in the amount of drift net loaded on

Taiwanese (lower graph) squid driftnet vessels fishing in the

1987 North Pacific Ocean. Data from NFRDA (1988) and Tung

(1988).

1000

ROK Vessels

800

Q)

600 c;

.....

0

(/) 400 c:

co

I200

0

1980 1982 1984 1986

1000 Taiwanese Vessels

- 800 Q)

.£

600 .....

0

(/)

c:

co 400

I200

0

1983 1985
__________________________ IgneU: The Fisheries for Neon Flying Squid 105

the foregoing directives are discussed. Production

statistics are to be reported monthly to the KDSFA,

and at season's end all squid vessels are to report

catch and effort statistics to NFRDA.

Gear and Vessel Characteristics

Most ROK driftnet vessels are converted tuna longliners, reflecting the economic difficulties faced by

tuna longliners in past years (Gong 1985). Vessels

range from 170 to 500 gt (Table 6); average displacement is approximately 290 gt, and about 90% of the

converted vessels are at least 16 years old (NFRDA,

unpub!. data on file with author).

Driftnet mesh sizes range from 86 to 155 mm (Y.

Gong, NFRDA, Pusan, Republic of Korea, pers.

commun., Nov. 1987); the most common sizes range

from 96 to 115 mm (Gong 1985). Single panels of

net (poks) are 50 m long and 9 m deep (Gooder

1989). Due to seasonal and geographical differences

in the size of flying squid (Gong, Kim, and Kim 1985;

Gooder 1989), mesh sizes are changed throughout

the fishing season. In the western grounds, 105- and

115-mm mesh gear is most common, whereas 86- and

96-mm mesh gear is most common' in the eastern

grounds (Gooder 1989).

Since the start of the fishery, the average number

of poks deployed nightly per vessel has risen steadily,

from 200 in 1980 to almost 1000 in 1986 (Fig. 3).

Little additional increase is expected, because most

ROK vessels are operating at full capacity if 1000

poks are deployed (Y. Gong, NFRDA, Pusan, Republic of Korea, pers. commun., Nov. 1987).

The Taiwanese Squid Driftnet Fishery _

Fishery Characteristics

The Taiwanese driftnet fishing grounds for flying

squid in the North Pacific Ocean were first developed

in 1977 by jigging vessels (Chen 1985; Tung 1988).

Increased oil prices in the late 1970s prompted 12

vessels to switch to driftnet gear in 1980, thereby saving on the oil costs necessary to maintain lights for

jigging (Tung 1988). The driftnet fishery grew to

over 100 vessels by 1983, and 166 vessels by 1988

(Table 1). Annual catches increased from 908 t in

1980 to a peak of 27,553 t in 1984, and then declined

to 18,578 tin 1987. Although catch per vessel per day

increased sligh tly between 1983 and 1987 (Table 3),

the number of nets deployed per vessel rose markedly (Table 7), resulting in a reduction over time of

catch per net.

Although fishing grounds were located initially in

the western North Pacific, the fishery extended eastward, reaching 1600 W by 1983 (Liu 1984) and 145 by

1986 (Tung 1988). Fishing has generally occurred

from May through October, with peak effort (vessel

days) in August and September. About 30% of the

total catch comes from fishing grounds east of

1700 E (Tung 1988). In early fall, vessels return westward, fishing near Japan's coast at season's end in

November.

Table 7

Arnounta of drift net (each piece equals 50 m) loaded on Taiwanese squid driftnet vessels fishing in the North Pacific

Ocean, 1983-87.

Pieces of net

200-399

400-599

600-799

800-999

1000-1I99

1200-1399

1400-1599

1600-1800

Total vessels:

Total nets:

a Data from Tung (1988).

1983

12

31

26

15

17

101

69500

Number of vessels

1984 1985 1986 1987

7 5 6 2

42 20 17 4

33 33 33 14

30 30 22 14

23 23 21 27

II 12 8 12

I I 10

2 4

146 124 1I0 87

II 2800 104000 91600 92100
106 NOAA Technical Report #105 _

Fishing Regulations

Because of U.S. concerns about the incidental harvest of salmonids, the Taiwanese adopted additional

domestic regulations in 1986 for squid driftnetters.

Under these regulations, fishing requires a permit

from the Council of Agriculture (CCNAA [Coordination Council for North American Affairs] 1989,

unpub\. data on file with author), and is prohibited

north of 39°N west of 170°£ (NMFS 1988). Regulations similar to Japanese domestic regulations were

adopted for waters east of 170°£ (Fig. 1). Since all

Taiwanese squid driftnetters operate from the Port of

Kaohsiung, Port authorities there are charged with

enforcement of fishing regulations (CCNAA, unpub\.

data on file with author).

Gear and Vessel Characteristics

In 1982, three years after the start of the squid

driftnet fishery, drift nets became the gear most commonly used by Taiwanese squid fishermen (Chen

1985). The overwhelming effectiveness and efficiency

of drift nets quickly became apparent, and by 1984

over 95% of the squid fleet were driftnetters (Tung

1988).

Most Taiwanese driftnet vessels range from 200 to

500 gt (Table 6); average displacement in 1987 was

325 gt. In recent years, the number of vessels ~700 gt

has been increasing. Taiwanese driftnet vessels are

typically converted longliners (Chen 1985).

Tung (1988) gives the following description of

mesh sizes used. Driftnet vessels carry a variety of

mesh sizes and may target both squid and tuna during a fishing season. Mesh sizes <120 mm are typically

used to harvest oceanic squid; the primary mes}: sizes

in this range are 110 mm and 80-85 mm. The recent

shift to smaller mesh sizes reportedly stems from

management guidelines in the western fishing

grounds that restrict fishing south of 39°N latitude,

thereby forcing fishing operations into areas where

squid are smal\. Although drift nets with mesh sizes

up to 160 mm can be used to harvest flying squid,

mesh sizes above 146 mm are used for harvesting

other species such as tuna and billfish in the largemesh driftnet fishery. Principal mesh sizes for the

large-mesh fishery range from 196 to 220 mm.

Similar to the ROK fishery, total driftnet length

used per vessel has increased yearly. At first, Taiwanese driftnetters were equipped with 250-500 tans of

drift nets (Anonymous 1981). By 1987, an average of

1059 tans of drift nets were loaded on each vessel

(Table 7; Fig. 3). About 50-75% of the nets (25-37

km) loaded aboard the vessel are fished nightly; this

percentage varies according to weather conditions

and the skipper's skill (I. Tung, National Taiwan University, Taipei, pers. commun.,Jan. 1988).

Overview of North Pacific Squid

Driftnet Fisheries _

Characteristics of Fishing Area

The northern portion of the flying squid fishing area

(where most of the fishing occurs) lies within what

may be the most important major frontal zone in the

North Pacific Ocean. Called the subarctic frontal

zone (SFZ) , this area is typified by multiple fronts

(mesoscale fronts) within a general frontal zone

ranging from 35° to 47°N (Roden 1977; Fig. 4). To

the north, the SFZ is bounded by subarctic waters

(cool, low salinity), with a permanent halocline between 100 and 200 m and a large seasonal

temperature variation; to the south are the transitional region and subtropical waters (stable, warm,

high salinity) with a permanent thermocline and no

halocline (Roden and Robinson 1988).

The chemical and biological properties of the SFZ

provide a unique environment to pelagic species.

The high productivities resulting from frontal dynamics influence forage aggregations, thereby

impacting species distributions in higher trophil; levels (McGowan 1986). For example, when frontal

structures are poorly developed and water-mass

boundaries indistinct, catches of albacore decline

(Laurs and Lynn 1977). This may explain why ROK

squid driftnet fishermen select areas of sharp frontal

gradients for net deployment (Gooder 1989). Japanese squid driftnet fishermen probably do likewise,

since sea-surface temperatures may differ by 2-3°C

from one end of a drift net to the other (Tsunoda

1989) .

As an environmental barrier, the SFZ affects size

and sexual dimorphism in flying squid distribution,

thereby affecting fishing patterns. Four modes in the

frequency distribution of mantle lengths have been

identified by Murakami et a\. (1981). Called the LL

(extra-large), L (large), S (small), and SS (extrasmall) groups, these modes presumably reflect

differences in age structure, spawning period, or sex,

and indicate possible differences in stocks or in lifehistory strategies.

The LL squid are found north of the SFZ, are primarily females, and are targeted by fishermen from

all three countries. Abundances of the LL group are

relatively high east of 170°£ and low west of 170°£

(Murata 1987). The L-, S-, and SS-group squid have
__________________________ Ignell: The Fisheries for Neon Flying Squid 107

SLBTROPICAL DOMAIN

.. '~. "1: .,-,.-.

160E 180 160W 140W 120W

~SFZ

~STFZ

120E 140E

20N

30N

50N

40N

Figure 4

Schematic diagram showing the locations of the Subarctic and Subtropical Domains, the North Pacific Transition Zone,

and the associated Subarctic and Subtropical Frontal Zones (SFZ and STFZ). Black arrows indicate intrusions of boundary

currents.

sex ratios near 1:1 (Kubodera et al. 1983), and are

sometimes targeted by ROK and Taiwanese

fishermen. The L- and S-group squid are distributed in and south of the subarctic boundary but

north of the SS-group (Murata 1987). Positive correlation between latitude and size is not restricted to

the four squid groups; it also pertains to squid within

a particular group (Kubodera et al. 1983), and to

other fishes such as pomfret that migrate seasonally

from subtropical to subarctic waters (Shimazaki

1986) and are incidentally harvested by driftnet

fishermen.

Fishing Methods

Japan, the ROK, and presumably Taiwan use similar

fishing gear and methods to harvest flying squid in

the North Pacific Ocean. The following paragraphs

describe a typical squid driftnet fishing operation,

summarized from Cary and Burgner (1983), Ignell et

al. (1986), Gooder (1989), and Tsunoda (1989).

Driftnet vessels often fish in a coordinated fashion:

nets are set along an east-west direction and set positions are spaced about 3.5 km apart (inter-vessel

distance) on a north-south axis. Information is exchanged among vessels to locate fishing grounds with

high squid abundance. Surface water temperatures

are monitored to determine regions of abrupt temperature change. Satellite produced, 5-day summaries of sea-surface conditions and other cues, such

as plankton and seabird densities, are also used to

position the vessels.

Drift nets are typically deployed in the afternoon,

usually 2-4 hours before sunset, along an east-west

direction. The net is deployed from the stern net well

over a horizontal roller at constant speed (near cruising speed). Sprayed water keeps the net from

tangling, and two men use poles to separate the

leadline and corkline. A combination of float, radio,

and light buoys marks the end of net sections.

Net retrieval begins before sunrise; incandescent

and halogen lights illuminate the deck, and a spotlight is directed at the net in the water. The net is

retrieved and hauled aboard the port side at speeds

from 1 to 5 knots. After the catch is removed, the net

is pulled through a tube on the starboard side and

back to the net well at the stern where it is stacked

for subsequent deployment. Billfish, large sharks,

and porpoise are often gaffed and pulled onto the

deck manually or with an electric winch.

Besides flying squid, species that may be retained

include albacore (Thunnus alalunga), skipjack

(Euthynnus pelamis), other tunas (Thunnus spp.),

swordfish (Xiphias gladius), striped marlin

(Tetrapturus audax), dolphin fish (Coryphaena
108 NOAA Technical Report #105 _

hippurus), and yellowtail (Seriola spp.). Others are

usually discarded, although large Pacific pomfret

(Brama japonica), ocean sunfish (Mola mola), and

salmon shark (Lamna ditropis) are occasionally kept.

During retrieval, approximately 8-10% of the squid

drop out of the net alongside the vessel. Dropout

rates for many of the non-commercial species caught

in the gear are considerably higher since they are

shaken out before they reach the deck.

Dorsal mantle lengths of commercially caught

squid range from 20 to >50 cm; most are at least 35

cm. Mantle-length modes for the ROK fishery are 38-

39 cm in the central North Pacific Ocean from June

to July, and 30-31 cm in the northwestern Pacific

Ocean from September to December (Gong, Kim,

and Kim 1985). About 95% of the squid in the Taiwanese commercial catch exceed 35 cm (Tung 1988).

The Japanese fishery is particularly selective towards

large squid because their regulations prohibit use of

nets with mesh sizes <100 mm, and because they prefer to fish north of the Taiwanese and ROK fleets

where the largest squid are found. Squid are processed by separating the mantle from the head and

by removing the arms, tentacles, and viscera. Mantles

(and on some vessels the head, arms, and fins also)

are placed in a pan and frozen.

Total Fishing Effort

Since 1983, the total number of squid driftnet vessels

licensed by Japan, Taiwan, and the ROK has ranged

from 715 vessels in 1983 to 779 vessels in 1988

(Fig. 5). Although fishing effort (measured in terms

300 fZZ3 Korea

Cii Japan

-0

I:: - ro Taiwan

(/)

:::l 200 0

I-

-(/)

I::

0

I- 100 .Q....

-

~.

0

1978 1980 1982 1984 1986

-Taiwan I'ZZJ Korea

800

Japan

(/) r«

Q)

(/) 600 (/)

Q)

>

-0

.... 400 Q)

.0

E

:::l

Z

200 Figure 5

Catch (t) and fishing effort (number of

vessels) for the squid driftnet fisheries of

0 Japan, ROK, and Taiwan in the North Pa1982 1984 1986 1988 cific Ocean. Fishing effort prior to 1981 is

unavailable for the Japanese fleet.
___________________________ IgneU: The Fisheries for Neon Flying Squid 109

of panels of drift net) is unavailable for the Taiwanese and Japanese fisheries, total fishing effort has

probably increased in recent years, owing to longer

fishing seasons and increased gear deployment. For

instance, since 1982 the average number of days

fishing per vessel has increased 19% in the Taiwanese

fishery, 11 % in the Japanese fishery, and 54% in the

ROK fishery (Table 1). The increase in gear deployment is most evident in the ROK and Taiwanese

driftnet fleets: since 1983, the average length of drift

net used nightly has increased 85% and 54% (Gong

1985; Tung 1988; Fig. 3), respectively. Because small

vessels fish 450 tans and large vessels fish 1200 tans

nightly, length of drift net deployed nightly in the

Japanese fleet has probably also increased because

fewer small vessels participate in the fishery.

Total Catch

Despite increased fishing effort and power, total

catches of flying squid have remained relatively

stable, ranging since 1982 from 200,000 to 300,000 t

(Fig. 5), and averaging 258,404 t since 1984 (Table

1). All Japanese and approximately 30% of ROK and

Taiwanese driftnet catches (about 75% of the total

flying squid catch) have occurred east of 1700E. Estimated catches of other retained species are

7500-23,000 t of albacore and probably several thousand metric tons each of skipjack, yellowtail, and

marlin (J. Wetherall, Natl. Mar. Fish. Serv., NOAA,

Honolulu, HI 96822-2396, pers. commun.).

Sustainable Yield

Total catch of flying squid in the North Pacific Ocean

is near or below estimated sustainable-yield levels.

The amount of flying squid caught by jigging vessels

in recent years is unknown; however, 29,000 t were

caught in 1984 (Anonymous 1986). Assuming that

current jigging harvests approach 35,000 t and that

62,500 t (25% of 250,000 t) of squid are caught by

driftnet vessels fishing west of 1700 E, total catch of

flying squid west of 1700 E is about 100,000 t, o~ 110

million squid (conversion data from Gong, Lim, and

H ur [1985]). This catch estimate is near Osako and

Murata's (1983) recommended catch level of 80,000-

100,000 t, but much less than Okutani's (1977)

potential catch of 150,000-600,000 t, The catch estimate also yields a relatively low rate of exploitation

«0.5) based on Chikuni's (1985) estimate of

250,000-350,000 t (estimate includes 2 less abundant

species: Onychoteuthis borealijaponica and Gonatopsis borealis) and Araya's (1983) estimate of 219-264 million

individuals for the flying squid stock west of 170°E.

A yield estimate has not been calculated for the

entire North Pacific Ocean flying squid resource;

however, total catch is probably within recommended

levels. The total catch of flying squid west of 1600 W

has averaged about 255,000 t from 1983 to 1985 (assuming all the ROK and Taiwanese, and 82% of the

Japanese catch occurs here). This estimate is probably within sustainable yield levels, based on the

estimate of Gong, Lim, and Hur (1985) of 322 and

469 million squid or 351,000-511,000 t for the flying

squid resource west of 1600 W.

Although recent harvests have apparently been

within sustainable yield levels, CPUE (one unit equals

one piece of net) has been declining. CPUE calculated from ROK commercial fishery logbook data has

fallen 75% since 1980 (Fig. 2). Despite yearly increases in fishing power, catches have remained

relatively stable (Table 1), suggesting that CPUE has

declined in recent years.

This decline may reflect a reduction in the largesized squid population in the western portion of the

fishery. In the late 1970s and early 1980s, mantle

lengths of squid caught by jigging vessels decreased

(Osako and Murata 1983). This decrease occurred

during a period of heavy exploitation (1978 jigging

catch was 151,000 t [Araya 1983]), and led Araya

(1983) to suggest that the winter spawning group was

"in danger and susceptible to overfishing." This decline could also prompt a change in targeting from

L- to S-sized squid, which would lead to additional

reductions in a CPUE index based on weight if abundances among groups were similar. On the other

hand, the decline may be indicative of gear competition in the fishery; gear competition occurs when

each additional unit of drift net, above some threshold, competes with preceding driftnet units

(Rothschild 1977). The data in Table 1 indicate that

years with low CPUE are weakly associated with years

of high driftnet effort, but more-detailed effort data

are needed to further test this hypothesis.

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